protected Expr ParseExpr(string s)
{
Expr e = ParseTerm(s);
while (pos < s.Length && (s[pos] == '+' || s[pos] == '-'))
{
char op = s[pos];
Consume(s, op);
Expr e2 = ParseTerm(s);
if (op == '+')
{
e = nf.Add(e, e2);
}
else if (op == '-')
{
e = nf.Subtract(e, e2);
}
}
return(e);
}