trait IterExtra: Iterator {
    fn all_eq(&mut self) -> bool
    where
        <Self as Iterator>::Item: std::cmp::PartialEq,
    {
        let first = match self.next() {
            Some(a) => a,
            None => return true,
        };
        for a in self {
            if a != first {
                return false;
            }
        }
        true
    }
}
impl<I: Iterator> IterExtra for I {}