static SS zipTwoListsToCIMap_strict(Cl<S> c1, Cl<S> c2) {
  int n = l(c1);
  if (n != l(c2)) null;
  LS l1 = asList(c1), l2 = asList(c2);
  SS map = ciMap();
  for i to n:
    if (!strictPutIC(map, l1.get(i), l2.get(i)))
      null;
  ret map;
}