def process_stop(color_block):
    """Handle the "stop after" checkbox.

    The user wants the machine to pause after this patch.  This can
    be useful for applique and similar on multi-needle machines that
    normally would not stop between colors.

    In machine embroidery files, there's no such thing as an actual
    "STOP" instruction.  All that exists is a "color change" command
    (which libembroidery calls STOP just to be confusing).

    On multi-needle machines, the user assigns needles to the colors in
    the design before starting stitching.  C01, C02, etc are normal
    needles, but C00 is special.  For a block of stitches assigned
    to C00, the machine will continue sewing with the last color it
    had and pause after it completes the C00 block.

    That means we need to add an artificial color change instruction
    shortly before the current stitch so that the user can set that color
    block to C00.  We'll go back 3 stitches and mark the start of the C00
    block:
    """

    if len(color_block.stitches) >= 3:
        # make a copy of the stitch and set it as a color change
        stitch = color_block.stitches[-3].copy()
        stitch.color_change = True

        # mark this stitch as a "stop" so that we can avoid
        # adding tie stitches in ties.py
        stitch.stop = True

        # insert it after the stitch
        color_block.stitches.insert(-2, stitch)

    # and also add a color change on this stitch, completing the C00
    # block:

    stitch = color_block.stitches[-1].copy()
    stitch.color_change = True
    color_block.add_stitch(stitch)

    # reference for the above: https://github.com/lexelby/inkstitch/pull/29#issuecomment-359175447