clip:(A,B,C,D) where: 

 
   +----D-------B-----+----> +
   |    |       |     |
 A-|----+-------+     |
   |    |       |     |
   |    | clip  |     |
 C-+----+-------+     |
   |                  |  
   +------------------+ <- original area
   |
   \/
   +