>>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 0)
    0
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 1)
    0
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 5)
    2
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 6)
    2

def find_index(segmentation, stroke_id):
    """
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 0)
    0
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 1)
    0
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 5)
    2
    >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 6)
    2
    """
    for i, symbol in enumerate(segmentation):
        for sid in symbol:
            if sid == stroke_id:
                return i
    return -1