static <A> HashMap<A> twoListsToHashMap(L<A> l1, L<A> l2) {
  if (l(l1) != l(l2)) fail("Differing list size: " + l(l1) + "/" + l(l2));
  new HashMap<A> map;
  int n = l(l1);
  for i to n:
    map.put(l1.get(i), l2.get(i));
  ret map;
}

static <A> HashMap<A> twoListsToHashMap(A[] l1, A[] l2) {
  ret twoListsToHashMap(Arrays.asList(l1), Arrays.asList(l2));
}