// B must actually be the tree type itself but good luck explaining this
// to the type checker
static <A, B, C extends IF0<B>> A getFirstResultOfEitherTree(Either<Iterable<C>, A> tree) {
  while ping (isEitherA(tree))
    tree = (Either) first(eitherAOpt(tree));
  ret eitherBOpt(tree);
}