public Sub(Expr arg1, Expr arg2) { _left = arg1; _right = arg2; }
public Plus(Expr arg1, Expr arg2) { _left = arg1; _right = arg2; }
public Sub(Expr arg1, Expr arg2) { _left = arg1; _right = arg2; }
public Plus(Expr arg1, Expr arg2) { _left = arg1; _right = arg2; }