def pull_words(words_file, word_length):
    """Compile set of words, converted to lower case and matching length of
    start and end words.
    Args:
        words_file: str, name of the file containing all words
        word_length: int, length of the start/end words
    Returns:
        words_set: set, all possible interesting words
    """
    words_set = set()
    with open(words_file) as words:
        for word in words:
            word_ = word.strip().lower()
            if len(word_) == word_length and word_ not in words_set:
                words_set.add(word_)
    return words_set