Suppress Macro-related exception

org.aminb.alloy.catalyst/src/main/java/org/aminb/alloy/catalyst/SSRVisitor.java

@@ -10,6 +10,7 @@ import edu.mit.csail.sdg.ast.ExprBadJoin;
 import edu.mit.csail.sdg.ast.ExprBinary;
 import edu.mit.csail.sdg.ast.ExprCall;
 import edu.mit.csail.sdg.ast.ExprConstant;
+import edu.mit.csail.sdg.ast.ExprCustom;
 import edu.mit.csail.sdg.ast.ExprITE;
 import edu.mit.csail.sdg.ast.ExprLet;
 import edu.mit.csail.sdg.ast.ExprList;
@@ -162,6 +163,17 @@ public class SSRVisitor extends VisitReturn {
         return null;
     }
 
+    /** Visits a ExprCustom node */
+    @Override
+    public Object visit(ExprCustom x) {
+        /**
+         * Similar to ExprBadJoin case above, but for Macro.  Happens
+         * in models/builtin/puzzle/coloring/color-australia.als, with
+         * a 'Incomplete call on the macro "adjacent"' message.
+         */
+        return null;
+    }
+
     /** Visits an ExprUnary node. */
     @Override
     public Object visit(ExprUnary x) {