findStretchyOperator(MathMLElement* elem, StretchId id)
findStretchyOperator(MathMLElement* elem)
findStretchyOperator(MathMLElement* elem, StretchId id){  MathMLOperatorElement* op = findStretchyOperator(elem);  if (op == NULL) return NULL;  if (op->GetStretch() != id) return NULL;  return op;}