OperatorKey key = iter.next() ;
// if the same key doesn't exist in the second plan
// we've got a problem
if (plan2.getOperator(key) == null) {
Operator op1 = plan1.getOperator(key) ;
if (messages != null) {
messages.append(op1.getClass().getSimpleName()) ;
appendOpKey(op1.getOperatorKey(), messages) ;
messages.append(" doesn't exist") ;
if (plan2Name != null) {
messages.append(" in ") ;
messages.append(plan2Name) ;
messages.append("\n") ;