static Map mapFieldToField(S fieldA, S fieldB, Iterable l) {
  ret fieldToFieldIndex(fieldA, fieldB, l);
}

static <A, B> Map<A, B> mapFieldToField(S fieldA, Class<A> typeA, S fieldB, Class<B> typeB, Iterable l) {
  ret filterMap(mapFieldToField(fieldA, fieldB, l), (k, v) -> isInstanceX(typeA, k) && isInstance(typeB, k));
}