static <A, B> void mapPutStrictlyOrFail(Map<A, B> map, A key, B value) {
  if (!(map != null && key != null && value != null)) ret;
  B old = map.get(key);
  if (old == null) { map.put(key, value); ret; }
  if (eq(old, value)) ret;
  fail("Can't overwrite key " + key + " (old value: " + old + ", new value: " + value);
}