If, instead of a GEL-to-boundary collision, DoCollision() senses a
GEL-to-GEL collision, the system calls your collision routine with the
following two arguments:

  * Address of the VSprite that is the uppermost (or leftmost if y
    coordinates are identical) GEL of a colliding pair.

  * Address of the VSprite that is the lowermost (or rightmost if y
    coordinates are identical) GEL of the pair.