DEF BLOCK HELP
The  model is held in a linked list in memory and  when  drawing, 
the  program travels along the list from begining to end  drawing 
each point.  START BLOCK defines any points that follow the point 
nearest  the  key cursor as part of the block  until  either  the 
block  end is reached or the end is reached.  The block is  drawn 
light.   END  BLOCK does the same working backwards.  The GROW  & 
SHRINK  functions grow or shrink the block up or down the  linked 
list.  UNMARK BLOCK undefines the block so it is drawn as normal. 
DELETE  BLOCK  permanently removes the block from  the  model  by 
linking the blockends and freeing the memory in between.











