>>> list(locate(['a', 'b', 'c', 'b'], lambda x: x == 'b'))
        [1, 3]

    If *window_size* is given, then the *pred* function will be called with
    that many items. This enables searching for sub-sequences:

        >>> iterable = [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]
        >>> pred = lambda *args: args == (1, 2, 3)
        >>> list(locate(iterable, pred=pred, window_size=3))
        [1, 5, 9]

    Use with :func:`seekable` to find indexes and then retrieve the associated
    items:

        >>> from itertools import count
        >>> from more_itertools import seekable
        >>> source = (3 * n + 1 if (n % 2) else n // 2 for n in count())
        >>> it = seekable(source)
        >>> pred = lambda x: x > 100
        >>> indexes = locate(it, pred=pred)
        >>> i = next(indexes)
        >>> it.seek(i)
        >>> next(it)
        106

    r,