find the indexes of a sub-sequence:

        >>> from more_itertools import windowed
        >>> iterable = [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]
        >>> sub = [1, 2, 3]
        >>> pred = lambda w: w == tuple(sub)  # windowed() returns tuples
        >>> list(locate(windowed(iterable, len(sub)), pred=pred))
        [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

    )