public double EvaluaExpression()
{
double left = Term();
Token t = ts.get();
while (true)
{
switch (t.Kind)
{
case '+':
left += Term();
t = ts.get();
break;
case '-':
left -= Term();
t = ts.get();
break;
default:
ts.putback(t);
return(left);
}
}
}