public Plus(Expr arg1, Expr arg2) : base(arg1, arg2)
 {
 }
 public Multiply(Expr arg1, Expr arg2) : base(arg1, arg2)
 {
 }
 public BinaryExpr(Expr arg1, Expr arg2)
 {
     _left  = arg1;
     _right = arg2;
 }