public BinopExp(Exp l, Token op, Exp r)
{
Left = l;
Right = r;
ExpType = typeof(int);
switch(op.Type){
case TokenType.PLUS:
Oper = Operator.ADD;
break;
case TokenType.MINUS:
Oper = Operator.SUB;
break;
case TokenType.MUL:
Oper = Operator.MUL;
break;
case TokenType.DIVIDE:
Oper = Operator.DIVIDE;
break;
}
}