diff --git a/src/inference/equivalence/SemanticEquivalenceProofSystem.java b/src/inference/equivalence/SemanticEquivalenceProofSystem.java
index a2e8aa6..4472553 100644
--- a/src/inference/equivalence/SemanticEquivalenceProofSystem.java
+++ b/src/inference/equivalence/SemanticEquivalenceProofSystem.java
@@ -124,7 +124,7 @@
 	private Set<RDLTerm> applyRule4(SemanticEquivalenceRelation relation, Set<RDLTerm> existTerms) {
 		Map<Variable, RDLTerm> binding = new HashMap<>();
 		Map<Variable, OrderVariableConstraint> orderConstraint = new HashMap<>();
-		Set<RDLTerm> apperTerms = new HashSet<>();
+		Set<RDLTerm> appearTerms = new HashSet<>();
 		if (rule4_1.isMatchedBy(relation, binding, orderConstraint)) {
 			Set<SemanticEquivalenceRelation> results = rule4_2.substitute(binding, orderConstraint, existTerms);
 			for (SemanticEquivalenceRelation result : results) {
@@ -137,12 +137,12 @@
 				if (!linear.equals(result)) {
 					proofGraph.put(linear, result);
 				}
-				apperTerms.addAll(linear.getLeftSideHand().getSubTerms(EvaluatableTerm.class).values());
-				apperTerms.add(linear.getLeftSideHand());
-				apperTerms.addAll(linear.getRightSideHand().getSubTerms(EvaluatableTerm.class).values());
-				apperTerms.add(linear.getRightSideHand());
+				appearTerms.addAll(linear.getLeftSideHand().getSubTerms(EvaluatableTerm.class).values());
+				appearTerms.add(linear.getLeftSideHand());
+				appearTerms.addAll(linear.getRightSideHand().getSubTerms(EvaluatableTerm.class).values());
+				appearTerms.add(linear.getRightSideHand());
 			}
 		}
-		return apperTerms;
+		return appearTerms;
 	}
 }