To specify the region in the playfield that the system will use to define
the outermost limits of the GEL boundaries, you use these gelsinfo
members: topmost, bottommost, leftmost, and rightmost. the docollision()
routine tests these boundaries when determining boundary collisions within
this rastport.  they have nothing whatsoever to do with graphical
clipping.  Graphical clipping makes use of the RastPort's clipping
rectangle.

Here is a typical program segment that assigns the members correctly (for
boundaries 50, 100, 80, 240).  It assumes that you already have a rastport
structure pointer named myRastPort.

    myRastPort->GelsInfo->topmost    = 50;
    myRastPort->GelsInfo->bottommost = 100;
    myRastPort->GelsInfo->leftmost   = 80;
    myRastPort->GelsInfo->rightmost  = 240;

 parameters to your boundary collision routine 
 parameters to your inter-gel collision routines 
 handling multiple collisions