pub trait PeekableIterator: Iterator {
  // required method
  fn peek_with<T>(&mut self, func: impl for<'a> FnOnce(Option<&'a Self::Item>) -> T) -> T

  // provided methods
  fn peek_map<T>(&mut self, func: impl for<'a> FnOnce(&'a Self::Item) -> T) -> Option<T>
  fn next_if(&mut self, func: impl FnOnce(&Self::Item) -> bool) -> Option<Self::Item>
  fn next_if_eq<T>(&mut self, expected: &T) -> Option<Self::Item>
  where
    Self::Item: PartialEq<T>,
    T: ?Sized,
}