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