def add_word_listener(word, callback, triggers=['space'], match_suffix=False, timeout=2):
    """
    Invokes a callback every time a sequence of characters is typed (e.g. 'pet')
    and followed by a trigger key (e.g. space). Modifiers (e.g. alt, ctrl,
    shift) are ignored.

    - `word` the typed text to be matched. E.g. 'pet'.
    - `callback` is an argument-less function to be invoked each time the word
    is typed.
    - `triggers` is the list of keys that will cause a match to be checked. If
    the user presses some key that is not a character (len>1) and not in
    triggers, the characters so far will be discarded. By default the trigger
    is only `space`.
    - `match_suffix` defines if endings of words should also be checked instead
    of only whole words. E.g. if true, typing 'carpet'+space will trigger the
    listener for 'pet'. Defaults to false, only whole words are checked.
    - `timeout` is the maximum number of seconds between typed characters before
    the current word is discarded. Defaults to 2 seconds.

    Returns the event handler created. To remove a word listener use
    `remove_word_listener(word)` or `remove_word_listener(handler)`.

    Note: all actions are performed on key down. Key up events are ignored.
    Note: word mathes are **case sensitive**.
    """
    state = _State()
    state.current = ''
    state.time = -1

    def handler(event):
        name = event.name
        if event.event_type == KEY_UP or name in all_modifiers: return

        if timeout and event.time - state.time > timeout:
            state.current = ''
        state.time = event.time

        matched = state.current == word or (match_suffix and state.current.endswith(word))
        if name in triggers and matched:
            callback()
            state.current = ''
        elif len(name) > 1:
            state.current = ''
        else:
            state.current += name

    hooked = hook(handler)
    def remove():
        hooked()
        del _word_listeners[word]
        del _word_listeners[handler]
        del _word_listeners[remove]
    _word_listeners[word] = _word_listeners[handler] = _word_listeners[remove] = remove
    # TODO: allow multiple word listeners and removing them correctly.
    return remove