Fixing small javadoc typo.
@@ -72,7 +72,7 @@ import java.util.Set;
/**
* Base class for all (first order, i.e., not ground) arithmetic rules.
*
- * Full equality checks (when two rules are the equal, but not the same refernce) are epensive.
+ * Full equality checks (when two rules are the equal, but not the same reference) are expensive.
*
* @author Stephen Bach
*/
