def matrix_to_lines(matrix, x, y, incby=1):
    """    Converts the `matrix` into an iterable of ((x1, y1), (x2, y2)) tuples which
    represent a sequence (horizontal line) of dark modules.

    The path starts at the 1st row of the matrix and moves down to the last
    row.

    :param x: Initial position on the x-axis.
    :param y: Initial position on the y-axis.
    :param incby: Value to move along the y-axis (default: 1).
    :rtype: iterable of (x1, y1), (x2, y2) tuples
    """
    y -= incby
    last_bit = 1
    for row in matrix:
        x1 = x
        x2 = x
        y += incby
        for bit in row:
            if last_bit != bit and not bit:
                yield (x1, y), (x2, y)
                x1 = x2
            x2 += 1
            if not bit:
                x1 += 1
            last_bit = bit
        if last_bit:
            yield (x1, y), (x2, y)
            last_bit = 0
