diff --git a/src/main/java/org/logicng/backbones/Backbone.java b/src/main/java/org/logicng/backbones/Backbone.java
index 54ceccb9..eaca5a89 100644
--- a/src/main/java/org/logicng/backbones/Backbone.java
+++ b/src/main/java/org/logicng/backbones/Backbone.java
@@ -57,10 +57,13 @@
  * Therefore these variables can be assigned to true or false.
  * </ol>
  * All variable sets which were not computed are empty.
- * @version 2.0.0
+ * @version 2.3.1
  * @since 1.5.0
  */
 public final class Backbone {
+
+    private static final Backbone UNSAT_BACKBONE = new Backbone(false, null, null, null);
+
     private final boolean sat;
     private final SortedSet<Variable> positiveBackbone;
     private final SortedSet<Variable> negativeBackbone;
@@ -98,7 +101,7 @@ public static Backbone satBackbone(final SortedSet<Variable> positiveBackbone, f
      * @return the backbone
      */
     public static Backbone unsatBackbone() {
-        return new Backbone(false, null, null, null);
+        return UNSAT_BACKBONE;
     }
 
     /**
diff --git a/src/main/java/org/logicng/util/FormulaRandomizerConfig.java b/src/main/java/org/logicng/util/FormulaRandomizerConfig.java
index a703a600..938ad7dd 100644
--- a/src/main/java/org/logicng/util/FormulaRandomizerConfig.java
+++ b/src/main/java/org/logicng/util/FormulaRandomizerConfig.java
@@ -46,7 +46,7 @@
  *     <li>maximum numbers of operands for conjunctions, disjunctions, PBCs, and CCs</li>
  * </ul>
  * Note that the weights can only be applied for inner nodes of the generated formula,
- * since the 'leafs' of a formula in LogicNG are <b>always</b> literals or PBCs. So the weight of literals and PBCs
+ * since the 'leaves' of a formula in LogicNG are <b>always</b> literals or PBCs. So the weight of literals and PBCs
  * will effectively be higher and the weights of all other formula types (especially conjunctions and disjunctions)
  * will be lower. Similarly, the weight of constants will usually be lower, because they are always reduced in LogicNG
  * unless they are a single formula.
@@ -278,8 +278,8 @@ public Builder weightNegativeLiteral(final double weightNegativeLiteral) {
         }
 
         /**
-         * Sets the relative weight of a disjuction.
-         * @param weightOr the relative weight of a disjuction
+         * Sets the relative weight of a disjunction.
+         * @param weightOr the relative weight of a disjunction
          * @return the builder
          */
         public Builder weightOr(final double weightOr) {
diff --git a/src/test/java/org/logicng/backbones/BackboneTest.java b/src/test/java/org/logicng/backbones/BackboneTest.java
index e257d84b..8ef56d8f 100644
--- a/src/test/java/org/logicng/backbones/BackboneTest.java
+++ b/src/test/java/org/logicng/backbones/BackboneTest.java
@@ -47,7 +47,7 @@
 
 /**
  * Unit tests for {@link Backbone}.
- * @version 2.0.0
+ * @version 2.3.1
  * @since 1.5.0
  */
 public class BackboneTest {
diff --git a/src/test/java/org/logicng/datastructures/AssignmentTest.java b/src/test/java/org/logicng/datastructures/AssignmentTest.java
index b5b93de2..39600551 100644
--- a/src/test/java/org/logicng/datastructures/AssignmentTest.java
+++ b/src/test/java/org/logicng/datastructures/AssignmentTest.java
@@ -44,7 +44,7 @@
 
 /**
  * Unit tests for the class {@link Assignment}.
- * @version 2.0.0
+ * @version 2.3.1
  * @since 1.0
  */
 public class AssignmentTest extends TestWithExampleFormulas {