#![feature(generic_associated_types)]

trait Is<T> where T: ?Sized {}

impl<T> Is<T> for T {}

trait FMapFamily {
    type Sub<H>: FMap<Holding=H>;
    fn fmap_sub<H, F, I>(subord: Self::Sub<H>, map: F) -> Self::Sub<I> where
        F: FnOnce(H) -> I;
}

trait FMap {
    type Holding;
    type Parent: FMapFamily where <<Self as FMap>::Parent as FMapFamily>::Sub<Self::Holding>: Is<Self>;
}