def uses_only(word, allowed):
    """Predicate that asks whether word *only* uses letters allowed string.
    word: string to be searched
    allowed: string of allowed letters
    """
    # This is a set intersection problem again, but I'll use string methods.
    # Cycle through letters in word this time
    for letter in word:
        # If a letter in word is not in allowed, it's not obeying the rules
        # so we need to return False
        if allowed.find(letter)==-1:
            return False
    return True