Method pop

code/chapter03/infix_to_postfix.rs:22–26

(&mut self)

    }

    fn pop(&mut self) -> Option<T> {
        if self.top == 0 { return None; }
        self.top -= 1;
        self.data.pop()
    }

    fn peek(&self) -> Option<&T> {
        if self.top == 0 { return None; }