Pivots an Iterable[K] into Map[K1, Iterable[K2]] by pivoting each K into (K1, K2) and grouping all instances of K2 for each K1 into an iterable.
Pivots an Iterable[K] into Map[K1, Iterable[K2]] by pivoting each K into (K1, K2) and grouping all instances of K2 for each K1 into an iterable.
"Uncurries" the supplied nested fn of K1 and K2 into a function that accepts a single K.