diff --git a/SPECIFICATIONS.md b/SPECIFICATIONS.md
index 857fec4..c0d5861 100644
--- a/SPECIFICATIONS.md
+++ b/SPECIFICATIONS.md
@@ -75,6 +75,7 @@ _predicates_ over the following types:
 - _byte array_
 - _date_
 - _boolean_
+- _null_
 - _set_ a deduplicated list of values of any type, except _variable_ or _set_
 
 While a Biscuit token does not use a textual representation for storage, we
@@ -117,6 +118,7 @@ We will represent the various types as follows:
 - byte array: `hex:01A2`
 - date in RFC 3339 format: `1985-04-12T23:20:50.52Z`
 - boolean: `true` or `false`
+- null: `null`, supported since block version 5
 - set: `[ "a", "b", "c"]`
 
 As an example, assuming we have the following facts: `parent("a", "b")`,
@@ -154,6 +156,8 @@ not equal, set inclusion.
 A _boolean_ is `true` or `false`. It supports the following operations:
 `==`, `!=`, `||`, `&&`, set inclusion.
 
+A _null_ is a default type indicating the absence of value. It supports the operations `==` and `!=` with any other type. _null_ is always equal to itself, and not equal to any other type
+
 A _set_ is a deduplicated list of terms of the same type. It cannot contain
 variables or other sets. It supports equal, not equal, , intersection, union,
 set inclusion.
@@ -184,13 +188,14 @@ The logic language is described by the following EBNF grammar:
 
 <predicate> ::= <name> "(" <sp>? <term> (<sp>? "," <sp>? <term> )* <sp>? ")"
 <term> ::= <fact_term> | <variable>
-<fact_term> ::= <boolean> | <string> | <number> | ("hex:" <bytes>) | <date> | <set>
-<set_term> ::= <boolean> | <string> | <number> | <bytes> | <date>
+<fact_term> ::= <boolean> | <string> | <number> | ("hex:" <bytes>) | <date> | <null> | <set>
+<set_term> ::= <boolean> | <string> | <number> | <bytes> | <date> | <null>
 
 
 <number> ::= "-"? [0-9]+
 <bytes> ::= ([a-z] | [0-9])+
 <boolean> ::= "true" | "false"
+<null> ::= "null"
 <date> ::= [0-9]* "-" [0-9] [0-9] "-" [0-9] [0-9] "T" [0-9] [0-9] ":" [0-9] [0-9] ":" [0-9] [0-9] ( "Z" | ( ("+" | "-") [0-9] [0-9] ":" [0-9] [0-9] ))
 <set> ::= "[" <sp>? ( <set_term> ( <sp>? "," <sp>? <set_term>)* <sp>? )? "]"
 
@@ -291,23 +296,27 @@ To validate an operation, all of a token's checks must succeed.
 
 One block can contain one or more checks.
 
-Their text representation is `check if` or `check all` followed by the body of the query.
+Their text representation is `check if`, `check all` or `reject if` followed by the body of the query.
 There can be multiple queries inside of a check, it will succeed if any of them
-succeeds. They are separated by a `or` token.
+succeeds (in the case of `reject if`, the check will fail if any query matches). They are separated by a `or` token.
 
 - a `check if` query succeeds if it finds one set of facts that matches the body and expressions
 - a `check all` query succeeds if all the sets of facts that match the body also succeed the expression.
-`check all` can only be used starting from block version 4
+- a `reject if` query succeeds if no set of facts matches the body and expressions
+
+`check all` can only be used starting from block version 4.  
+`reject if` can only be used starting from block version 5.
 
 Here are some examples of writing checks:
 
 #### Basic token
 
 This first token defines a list of authority facts giving `read` and `write`
-rights on `file1`, `read` on `file2`. The first caveat checks that the operation
+rights on `file1`, `read` on `file2`. The first check ensures that the operation
 is `read` (and will not allow any other `operation` fact), and then that we have
-the `read` right over the resource.
-The second caveat checks that the resource is `file1`.
+the `read` right over the resource.  
+The second check ensures that the resource is either `file1` or `file2`.  
+The third check ensures that the resource is not `file1`.
 
 ```
 authority:
@@ -323,20 +332,24 @@ check if
 ----------
 Block 2:
 check if
-  resource("file1")  // restrict to file1 resource
+  resource("file1")  
+  or resource("file2") // restrict to file1 or file2
+----------
+Block 3:
+reject if
+  resource("file1")  // forbid using the token on file1
 ```
 
 The verifier side provides the `resource` and `operation` facts with information
 from the request.
 
-If the verifier provided the facts `resource("file2")` and
+If the verifier provided the facts `resource("file1")` and
 `operation("read")`, the rule application of the first check would see
-`resource("file2"), operation("read"), right("file2", "read")`
-with `X = "file2"`, so it would succeed, but the second check would fail
-because it expects `resource("file1")`.
+`resource("file1"), operation("read"), right("file1", "read")`
+with `X = "file1"`, so it would succeed, the second check would also succeed because it expects `resource("file1")` or `resource("file2")`. The third check would then fail because it would match on `resource("file1")`.
 
-If the verifier provided the facts `resource("file1")` and
-`operation("read")`, both checks would succeed.
+If the verifier provided the facts `resource("file2")` and
+`operation("read")`, all checks would succeed.
 
 #### Broad authority rules
 
@@ -659,7 +672,7 @@ each block must carry its own version.
   when possible, especially for biscuit versions that are only additive in terms of features.
 
 - The lowest supported biscuit version is `3`;
-- The highest supported biscuit version is `4`;
+- The highest supported biscuit version is `5`;
 
 # Version 2
 
diff --git a/samples/current/README.md b/samples/current/README.md
index e123c7c..e7a0ae1 100644
--- a/samples/current/README.md
+++ b/samples/current/README.md
@@ -45,7 +45,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -53,7 +53,7 @@ World {
             "resource(\"file1\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -68,7 +68,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -296,7 +296,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -305,7 +305,7 @@ World {
             "resource(\"file2\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -316,7 +316,7 @@ World {
             "user_id(\"alice\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 2,
@@ -328,7 +328,7 @@ World {
     },
 ]
   rules: [
-    AuthorizerRuleSet {
+    Rules {
         origin: Some(
             1,
         ),
@@ -338,7 +338,7 @@ World {
     },
 ]
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -407,7 +407,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -416,7 +416,7 @@ World {
             "resource(\"file2\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -426,7 +426,7 @@ World {
             "right(\"file1\", \"read\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 2,
@@ -439,7 +439,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -499,7 +499,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -512,7 +512,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -574,7 +574,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -583,7 +583,7 @@ World {
             "resource(\"file2\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -593,7 +593,7 @@ World {
             "right(\"file1\", \"read\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -606,7 +606,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -657,7 +657,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -666,7 +666,7 @@ World {
             "resource(\"file2\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -679,7 +679,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -728,7 +728,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -740,7 +740,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -773,7 +773,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -785,7 +785,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -847,7 +847,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -856,7 +856,7 @@ World {
             "time(2020-12-21T09:23:12Z)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
             Some(
@@ -867,7 +867,7 @@ World {
             "valid_date(\"file1\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -880,7 +880,7 @@ World {
     },
 ]
   rules: [
-    AuthorizerRuleSet {
+    Rules {
         origin: Some(
             1,
         ),
@@ -891,7 +891,7 @@ World {
     },
 ]
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -925,7 +925,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -934,7 +934,7 @@ World {
             "time(2020-12-21T09:23:12Z)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -947,7 +947,7 @@ World {
     },
 ]
   rules: [
-    AuthorizerRuleSet {
+    Rules {
         origin: Some(
             1,
         ),
@@ -958,7 +958,7 @@ World {
     },
 ]
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -1006,7 +1006,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1017,7 +1017,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1049,7 +1049,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1060,7 +1060,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1108,7 +1108,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1121,7 +1121,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -1177,7 +1177,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -1190,7 +1190,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1214,7 +1214,7 @@ result: `Err(FailedLogic(Unauthorized { policy: Allow(0), checks: [Block(FailedB
 ### token
 
 authority:
-symbols: ["hello world", "hello", "world", "aaabde", "a*c?.e", "abd", "aaa", "b", "de", "abcD12", "abc", "def"]
+symbols: ["hello world", "hello", "world", "aaabde", "a*c?.e", "abd", "aaa", "b", "de", "abcD12", "é", "abc", "def"]
 
 public keys: []
 
@@ -1239,6 +1239,8 @@ check if "aaabde".matches("a*c?.e");
 check if "aaabde".contains("abd");
 check if "aaabde" == "aaa" + "b" + "de";
 check if "abcD12" == "abcD12";
+check if "abcD12".length() == 6;
+check if "é".length() == 2;
 check if 2019-12-04T09:46:41Z < 2020-12-04T09:46:41Z;
 check if 2020-12-04T09:46:41Z > 2019-12-04T09:46:41Z;
 check if 2019-12-04T09:46:41Z <= 2020-12-04T09:46:41Z;
@@ -1268,7 +1270,7 @@ allow if true;
 ```
 
 revocation ids:
-- `f61b4cb4fc58777fec6c8d39fe62259dc3c78511868236c391e9f67ffd03a3a8b8e3042d4bacce0d5756d053f5afccd4c5e4df0597af44b36bdfab492e5fe50e`
+- `3d5b23b502b3dd920bfb68b9039164d1563bb8927210166fa5c17f41b76b31bb957bc2ed3318452958f658baa2d398fe4cf25c58a27e6c8bc42c9702c8aa1b0c`
 
 authorizer world:
 ```
@@ -1276,7 +1278,7 @@ World {
   facts: []
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1287,7 +1289,9 @@ World {
             "check if \"aaabde\".contains(\"abd\")",
             "check if \"aaabde\".matches(\"a*c?.e\")",
             "check if \"abcD12\" == \"abcD12\"",
+            "check if \"abcD12\".length() == 6",
             "check if \"hello world\".starts_with(\"hello\") && \"hello world\".ends_with(\"world\")",
+            "check if \"é\".length() == 2",
             "check if (true || false) && true",
             "check if 1 + 2 * 3 - 4 / 2 == 5",
             "check if 1 < 2",
@@ -1400,7 +1404,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1408,7 +1412,7 @@ World {
             "operation(\"write\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
             Some(
@@ -1421,7 +1425,7 @@ World {
     },
 ]
   rules: [
-    AuthorizerRuleSet {
+    Rules {
         origin: Some(
             1,
         ),
@@ -1431,7 +1435,7 @@ World {
     },
 ]
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1492,7 +1496,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1501,7 +1505,7 @@ World {
             "resource(\"file1\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1516,7 +1520,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -1564,7 +1568,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1577,7 +1581,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -1652,7 +1656,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1692,7 +1696,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -1759,7 +1763,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1769,7 +1773,7 @@ World {
             "authority_fact(1)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -1782,7 +1786,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             2,
         ),
@@ -1843,7 +1847,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1853,7 +1857,7 @@ World {
             "right(\"read\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -1866,7 +1870,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1874,7 +1878,7 @@ World {
             "check if group(\"admin\") trusting ed25519/acdd6d5b53bfee478bf689f8e012fe7988bf755e3d7c5152947abc149bc20189",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -1924,7 +1928,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1933,7 +1937,7 @@ World {
             "operation(\"B\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -1946,7 +1950,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -1979,7 +1983,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             None,
         },
@@ -1988,7 +1992,7 @@ World {
             "operation(\"invalid\")",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -2001,7 +2005,7 @@ World {
 ]
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -2108,7 +2112,7 @@ authorizer world:
 ```
 World {
   facts: [
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 0,
@@ -2118,7 +2122,7 @@ World {
             "query(0)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -2128,7 +2132,7 @@ World {
             "query(1)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 1,
@@ -2141,7 +2145,7 @@ World {
             "query(1, 2)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 2,
@@ -2151,7 +2155,7 @@ World {
             "query(2)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 3,
@@ -2161,7 +2165,7 @@ World {
             "query(3)",
         ],
     },
-    AuthorizerFactSet {
+    Facts {
         origin: {
             Some(
                 4,
@@ -2173,7 +2177,7 @@ World {
     },
 ]
   rules: [
-    AuthorizerRuleSet {
+    Rules {
         origin: Some(
             1,
         ),
@@ -2183,7 +2187,7 @@ World {
     },
 ]
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -2191,7 +2195,7 @@ World {
             "check if true trusting previous, ed25519/acdd6d5b53bfee478bf689f8e012fe7988bf755e3d7c5152947abc149bc20189",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             1,
         ),
@@ -2200,7 +2204,7 @@ World {
             "check if query(2), query(3) trusting ed25519/a060270db7e9c9f06e8f9cc33a64e99f6596af12cb01c4b638df8afc7b642463",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             2,
         ),
@@ -2209,7 +2213,7 @@ World {
             "check if query(2), query(3) trusting ed25519/a060270db7e9c9f06e8f9cc33a64e99f6596af12cb01c4b638df8afc7b642463",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             3,
         ),
@@ -2218,7 +2222,7 @@ World {
             "check if query(2), query(3) trusting ed25519/a060270db7e9c9f06e8f9cc33a64e99f6596af12cb01c4b638df8afc7b642463",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             4,
         ),
@@ -2227,7 +2231,7 @@ World {
             "check if query(4) trusting ed25519/f98da8c1cf907856431bfc3dc87531e0eaadba90f919edc232405b85877ef136",
         ],
     },
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             18446744073709551615,
         ),
@@ -2280,7 +2284,7 @@ World {
   facts: []
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -2335,7 +2339,7 @@ World {
   facts: []
   rules: []
   checks: [
-    AuthorizerCheckSet {
+    Checks {
         origin: Some(
             0,
         ),
@@ -2357,3 +2361,298 @@ World {
 
 result: `Ok(0)`
 
+
+------------------------------
+
+## test reject if: test029_reject_if.bc
+### token
+
+authority:
+symbols: ["test"]
+
+public keys: []
+
+```
+reject if test($test), $test;
+```
+
+### validation
+
+authorizer code:
+```
+test(false);
+
+allow if true;
+```
+
+revocation ids:
+- `2060031eb9968b492123440fa9cbc781f18be812961e765a34a8702d3eee0ed54910710efbb41b3141f60748a815012fe0e703a5b5604f4262d1ac7e79766b07`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "test(false)",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "reject if test($test), $test",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Ok(0)`
+### validation for "rejection"
+
+authorizer code:
+```
+test(true);
+
+allow if true;
+```
+
+revocation ids:
+- `2060031eb9968b492123440fa9cbc781f18be812961e765a34a8702d3eee0ed54910710efbb41b3141f60748a815012fe0e703a5b5604f4262d1ac7e79766b07`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "test(true)",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "reject if test($test), $test",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Err(FailedLogic(Unauthorized { policy: Allow(0), checks: [Block(FailedBlockCheck { block_id: 0, check_id: 0, rule: "reject if test($test), $test" })] }))`
+
+
+------------------------------
+
+## test null: test30_null.bc
+### token
+
+authority:
+symbols: ["fact", "value"]
+
+public keys: []
+
+```
+check if fact(null, $value), $value == null;
+reject if fact(null, $value), $value != null;
+```
+
+### validation
+
+authorizer code:
+```
+fact(null, null);
+
+allow if true;
+```
+
+revocation ids:
+- `bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "fact(null, null)",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "check if fact(null, $value), $value == null",
+            "reject if fact(null, $value), $value != null",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Ok(0)`
+### validation for "rejection1"
+
+authorizer code:
+```
+fact(null, 1);
+
+allow if true;
+```
+
+revocation ids:
+- `bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "fact(null, 1)",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "check if fact(null, $value), $value == null",
+            "reject if fact(null, $value), $value != null",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Err(FailedLogic(Unauthorized { policy: Allow(0), checks: [Block(FailedBlockCheck { block_id: 0, check_id: 0, rule: "check if fact(null, $value), $value == null" }), Block(FailedBlockCheck { block_id: 0, check_id: 1, rule: "reject if fact(null, $value), $value != null" })] }))`
+### validation for "rejection2"
+
+authorizer code:
+```
+fact(null, true);
+
+allow if true;
+```
+
+revocation ids:
+- `bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "fact(null, true)",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "check if fact(null, $value), $value == null",
+            "reject if fact(null, $value), $value != null",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Err(FailedLogic(Unauthorized { policy: Allow(0), checks: [Block(FailedBlockCheck { block_id: 0, check_id: 0, rule: "check if fact(null, $value), $value == null" }), Block(FailedBlockCheck { block_id: 0, check_id: 1, rule: "reject if fact(null, $value), $value != null" })] }))`
+### validation for "rejection3"
+
+authorizer code:
+```
+fact(null, "abcd");
+
+allow if true;
+```
+
+revocation ids:
+- `bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003`
+
+authorizer world:
+```
+World {
+  facts: [
+    Facts {
+        origin: {
+            None,
+        },
+        facts: [
+            "fact(null, \"abcd\")",
+        ],
+    },
+]
+  rules: []
+  checks: [
+    Checks {
+        origin: Some(
+            0,
+        ),
+        checks: [
+            "check if fact(null, $value), $value == null",
+            "reject if fact(null, $value), $value != null",
+        ],
+    },
+]
+  policies: [
+    "allow if true",
+]
+}
+```
+
+result: `Err(FailedLogic(Unauthorized { policy: Allow(0), checks: [Block(FailedBlockCheck { block_id: 0, check_id: 0, rule: "check if fact(null, $value), $value == null" }), Block(FailedBlockCheck { block_id: 0, check_id: 1, rule: "reject if fact(null, $value), $value != null" })] }))`
+
diff --git a/samples/current/samples.json b/samples/current/samples.json
index 432c45d..4af6b9c 100644
--- a/samples/current/samples.json
+++ b/samples/current/samples.json
@@ -1239,12 +1239,13 @@
             "b",
             "de",
             "abcD12",
+            "é",
             "abc",
             "def"
           ],
           "public_keys": [],
           "external_key": null,
-          "code": "check if true;\ncheck if !false;\ncheck if !false && true;\ncheck if false || true;\ncheck if (true || false) && true;\ncheck if true == true;\ncheck if false == false;\ncheck if 1 < 2;\ncheck if 2 > 1;\ncheck if 1 <= 2;\ncheck if 1 <= 1;\ncheck if 2 >= 1;\ncheck if 2 >= 2;\ncheck if 3 == 3;\ncheck if 1 + 2 * 3 - 4 / 2 == 5;\ncheck if \"hello world\".starts_with(\"hello\") && \"hello world\".ends_with(\"world\");\ncheck if \"aaabde\".matches(\"a*c?.e\");\ncheck if \"aaabde\".contains(\"abd\");\ncheck if \"aaabde\" == \"aaa\" + \"b\" + \"de\";\ncheck if \"abcD12\" == \"abcD12\";\ncheck if 2019-12-04T09:46:41Z < 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z > 2019-12-04T09:46:41Z;\ncheck if 2019-12-04T09:46:41Z <= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2019-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z == 2020-12-04T09:46:41Z;\ncheck if hex:12ab == hex:12ab;\ncheck if [1, 2].contains(2);\ncheck if [2019-12-04T09:46:41Z, 2020-12-04T09:46:41Z].contains(2020-12-04T09:46:41Z);\ncheck if [false, true].contains(true);\ncheck if [\"abc\", \"def\"].contains(\"abc\");\ncheck if [hex:12ab, hex:34de].contains(hex:34de);\ncheck if [1, 2].contains([2]);\ncheck if [1, 2] == [1, 2];\ncheck if [1, 2].intersection([2, 3]) == [2];\ncheck if [1, 2].union([2, 3]) == [1, 2, 3];\ncheck if [1, 2, 3].intersection([1, 2]).contains(1);\ncheck if [1, 2, 3].intersection([1, 2]).length() == 2;\n"
+          "code": "check if true;\ncheck if !false;\ncheck if !false && true;\ncheck if false || true;\ncheck if (true || false) && true;\ncheck if true == true;\ncheck if false == false;\ncheck if 1 < 2;\ncheck if 2 > 1;\ncheck if 1 <= 2;\ncheck if 1 <= 1;\ncheck if 2 >= 1;\ncheck if 2 >= 2;\ncheck if 3 == 3;\ncheck if 1 + 2 * 3 - 4 / 2 == 5;\ncheck if \"hello world\".starts_with(\"hello\") && \"hello world\".ends_with(\"world\");\ncheck if \"aaabde\".matches(\"a*c?.e\");\ncheck if \"aaabde\".contains(\"abd\");\ncheck if \"aaabde\" == \"aaa\" + \"b\" + \"de\";\ncheck if \"abcD12\" == \"abcD12\";\ncheck if \"abcD12\".length() == 6;\ncheck if \"é\".length() == 2;\ncheck if 2019-12-04T09:46:41Z < 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z > 2019-12-04T09:46:41Z;\ncheck if 2019-12-04T09:46:41Z <= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2019-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z >= 2020-12-04T09:46:41Z;\ncheck if 2020-12-04T09:46:41Z == 2020-12-04T09:46:41Z;\ncheck if hex:12ab == hex:12ab;\ncheck if [1, 2].contains(2);\ncheck if [2019-12-04T09:46:41Z, 2020-12-04T09:46:41Z].contains(2020-12-04T09:46:41Z);\ncheck if [false, true].contains(true);\ncheck if [\"abc\", \"def\"].contains(\"abc\");\ncheck if [hex:12ab, hex:34de].contains(hex:34de);\ncheck if [1, 2].contains([2]);\ncheck if [1, 2] == [1, 2];\ncheck if [1, 2].intersection([2, 3]) == [2];\ncheck if [1, 2].union([2, 3]) == [1, 2, 3];\ncheck if [1, 2, 3].intersection([1, 2]).contains(1);\ncheck if [1, 2, 3].intersection([1, 2]).length() == 2;\n"
         }
       ],
       "validations": {
@@ -1262,7 +1263,9 @@
                   "check if \"aaabde\".contains(\"abd\")",
                   "check if \"aaabde\".matches(\"a*c?.e\")",
                   "check if \"abcD12\" == \"abcD12\"",
+                  "check if \"abcD12\".length() == 6",
                   "check if \"hello world\".starts_with(\"hello\") && \"hello world\".ends_with(\"world\")",
+                  "check if \"é\".length() == 2",
                   "check if (true || false) && true",
                   "check if 1 + 2 * 3 - 4 / 2 == 5",
                   "check if 1 < 2",
@@ -1307,7 +1310,7 @@
           },
           "authorizer_code": "allow if true;\n",
           "revocation_ids": [
-            "f61b4cb4fc58777fec6c8d39fe62259dc3c78511868236c391e9f67ffd03a3a8b8e3042d4bacce0d5756d053f5afccd4c5e4df0597af44b36bdfab492e5fe50e"
+            "3d5b23b502b3dd920bfb68b9039164d1563bb8927210166fa5c17f41b76b31bb957bc2ed3318452958f658baa2d398fe4cf25c58a27e6c8bc42c9702c8aa1b0c"
           ]
         }
       }
@@ -2174,6 +2177,330 @@
           ]
         }
       }
+    },
+    {
+      "title": "test reject if",
+      "filename": "test029_reject_if.bc",
+      "token": [
+        {
+          "symbols": [
+            "test"
+          ],
+          "public_keys": [],
+          "external_key": null,
+          "code": "reject if test($test), $test;\n"
+        }
+      ],
+      "validations": {
+        "": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "test(false)"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "reject if test($test), $test"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Ok": 0
+          },
+          "authorizer_code": "test(false);\n\nallow if true;\n",
+          "revocation_ids": [
+            "2060031eb9968b492123440fa9cbc781f18be812961e765a34a8702d3eee0ed54910710efbb41b3141f60748a815012fe0e703a5b5604f4262d1ac7e79766b07"
+          ]
+        },
+        "rejection": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "test(true)"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "reject if test($test), $test"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Err": {
+              "FailedLogic": {
+                "Unauthorized": {
+                  "policy": {
+                    "Allow": 0
+                  },
+                  "checks": [
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 0,
+                        "rule": "reject if test($test), $test"
+                      }
+                    }
+                  ]
+                }
+              }
+            }
+          },
+          "authorizer_code": "test(true);\n\nallow if true;\n",
+          "revocation_ids": [
+            "2060031eb9968b492123440fa9cbc781f18be812961e765a34a8702d3eee0ed54910710efbb41b3141f60748a815012fe0e703a5b5604f4262d1ac7e79766b07"
+          ]
+        }
+      }
+    },
+    {
+      "title": "test null",
+      "filename": "test30_null.bc",
+      "token": [
+        {
+          "symbols": [
+            "fact",
+            "value"
+          ],
+          "public_keys": [],
+          "external_key": null,
+          "code": "check if fact(null, $value), $value == null;\nreject if fact(null, $value), $value != null;\n"
+        }
+      ],
+      "validations": {
+        "": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "fact(null, null)"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "check if fact(null, $value), $value == null",
+                  "reject if fact(null, $value), $value != null"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Ok": 0
+          },
+          "authorizer_code": "fact(null, null);\n\nallow if true;\n",
+          "revocation_ids": [
+            "bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003"
+          ]
+        },
+        "rejection1": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "fact(null, 1)"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "check if fact(null, $value), $value == null",
+                  "reject if fact(null, $value), $value != null"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Err": {
+              "FailedLogic": {
+                "Unauthorized": {
+                  "policy": {
+                    "Allow": 0
+                  },
+                  "checks": [
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 0,
+                        "rule": "check if fact(null, $value), $value == null"
+                      }
+                    },
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 1,
+                        "rule": "reject if fact(null, $value), $value != null"
+                      }
+                    }
+                  ]
+                }
+              }
+            }
+          },
+          "authorizer_code": "fact(null, 1);\n\nallow if true;\n",
+          "revocation_ids": [
+            "bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003"
+          ]
+        },
+        "rejection2": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "fact(null, true)"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "check if fact(null, $value), $value == null",
+                  "reject if fact(null, $value), $value != null"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Err": {
+              "FailedLogic": {
+                "Unauthorized": {
+                  "policy": {
+                    "Allow": 0
+                  },
+                  "checks": [
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 0,
+                        "rule": "check if fact(null, $value), $value == null"
+                      }
+                    },
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 1,
+                        "rule": "reject if fact(null, $value), $value != null"
+                      }
+                    }
+                  ]
+                }
+              }
+            }
+          },
+          "authorizer_code": "fact(null, true);\n\nallow if true;\n",
+          "revocation_ids": [
+            "bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003"
+          ]
+        },
+        "rejection3": {
+          "world": {
+            "facts": [
+              {
+                "origin": [
+                  null
+                ],
+                "facts": [
+                  "fact(null, \"abcd\")"
+                ]
+              }
+            ],
+            "rules": [],
+            "checks": [
+              {
+                "origin": 0,
+                "checks": [
+                  "check if fact(null, $value), $value == null",
+                  "reject if fact(null, $value), $value != null"
+                ]
+              }
+            ],
+            "policies": [
+              "allow if true"
+            ]
+          },
+          "result": {
+            "Err": {
+              "FailedLogic": {
+                "Unauthorized": {
+                  "policy": {
+                    "Allow": 0
+                  },
+                  "checks": [
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 0,
+                        "rule": "check if fact(null, $value), $value == null"
+                      }
+                    },
+                    {
+                      "Block": {
+                        "block_id": 0,
+                        "check_id": 1,
+                        "rule": "reject if fact(null, $value), $value != null"
+                      }
+                    }
+                  ]
+                }
+              }
+            }
+          },
+          "authorizer_code": "fact(null, \"abcd\");\n\nallow if true;\n",
+          "revocation_ids": [
+            "bbf3ad51a70e935126b334f37be2bf66e90162353c19c524c0d3579ee71034996872b8433b132e6e0b519d371b0ab20481d58c4619183e8997c3744786e8e003"
+          ]
+        }
+      }
     }
   ]
 }
diff --git a/samples/current/test017_expressions.bc b/samples/current/test017_expressions.bc
index 10f5024..1f3234c 100644
Binary files a/samples/current/test017_expressions.bc and b/samples/current/test017_expressions.bc differ
diff --git a/samples/current/test029_reject_if.bc b/samples/current/test029_reject_if.bc
new file mode 100644
index 0000000..9d6c4d3
Binary files /dev/null and b/samples/current/test029_reject_if.bc differ
diff --git a/samples/current/test30_null.bc b/samples/current/test30_null.bc
new file mode 100644
index 0000000..2337a11
Binary files /dev/null and b/samples/current/test30_null.bc differ