start adding tests for functions

Tests/syntax_test_scss.scss

@@ -764,3 +764,20 @@ nav ul {
   }
 }
 
+//=============================================================================
+// Function
+// https://sass-lang.com/documentation/at-rules/function/
+//=============================================================================
+@function fibonacci($n) {
+  $sequence: 0 1;
+  @for $_ from 1 through $n {
+    $new: nth($sequence, length($sequence)) + nth($sequence, length($sequence) - 1);
+    $sequence: append($sequence, $new);
+  }
+  @return nth($sequence, length($sequence));
+}
+
+.sidebar {
+  float: left;
+  margin-left: fibonacci(4) * 1px;
+}