// returns case-insensitive trie iff map is case-insensitive
static <A> StringTrie<A> asStringTrie(Map<S, A> map) {
  new StringTrie<A> trie;
  if (isCIMap(map)) trie.makeCaseInsensitive();
  if (map != null)
    for (Map.Entry<S, A> e : map.entrySet())
      trie.add(e.getKey(), e.getValue());
  ret trie;
}