Add implementation of hashCode in VariableTerm

The class implements equals() so it should also implement hashCode().

Change-Id: Iea1a9dc9fdd480205f8ba0eb2e60a45f555acf91

src/lang/VariableTerm.java

diff --git a/src/lang/VariableTerm.java b/src/lang/VariableTerm.java
index 3aede4f..54b326b 100644
--- a/src/lang/VariableTerm.java
+++ b/src/lang/VariableTerm.java
@@ -197,6 +197,11 @@
 	return this == obj;
     }
 
+    @Override
+    public int hashCode() {
+      return System.identityHashCode(this);
+    }
+
     /**
      * Returns a string representation of this term if unbound.
      * Otherwise, returns the value of dereferenced term: