code: public AutoComplete autoCompleteDirectory(String prefix) {
          final Trie<CliValueType> possibilities = childDirectories.subTrie(prefix).mapValues(CliValueType.DIRECTORY.<CliDirectory>getMapper());
          return new AutoComplete(prefix, possibilities);
      }
docs: Auto complete the given prefix with child directory possibilities.
  
  @param prefix Prefix to offer auto complete for.
  @return Auto complete for child {@link CliDirectory}s that starts with the given prefix. Case insensitive.