Generate a sequence of filenames that match ``word``.

        :param word: the word to complete

def gen_filename_completions(self, word, filenames):
        """Generate a sequence of filenames that match ``word``.

        :param word: the word to complete
        """
        if not word:
            return filenames
        else:
            trie = pygtrie.CharTrie()
            for filename in filenames:
                trie[filename] = filename
            return trie.iterkeys(prefix=word)