static <A, B> ItIt<Map<A, B>> allPartialMappings(Cl<A> l1, Cl<B> l2) {
  L<A> _l1 = asList(l1);
  L<B> _l2 = asList(l2);
  ret (ItIt) nestedI(allSubsetsOfLength(l(l1), _l2), subset ->
    mapI(allPermutations(subset), perm -> zipTwoListsToOrderedMap(_l1, perm)));
}