@Override
 public <R, T> R accept(IAqlExpressionVisitor<R, T> visitor, T arg) throws AsterixException {
   return visitor.visitQuery(this, arg);
 }
 @Override
 public <R, T> R accept(IAqlExpressionVisitor<R, T> visitor, T arg) throws AsterixException {
   return visitor.visitCreateFeedPolicyStatement(this, arg);
 }
 @Override
 public <R, T> R accept(IAqlExpressionVisitor<R, T> visitor, T arg) throws AsterixException {
   return visitor.visitIndexDropStatement(this, arg);
 }