50. Novel screen clearing
    ~~~~~~~~~~~~~~~~~~~~~
These two procedures give different ways of clearing the screen of text and
graphics in Modes 0,1,2,4 & 5. The first wipes from the borders inwards,
rather like a fadeout on TV or films. The second clears the screen in a way
similar to curtains closing. You must alter line 110 slightly for Mode 0,
which will clear more slowly than the others. You can speed up the second
procedure, but not the first, in Modes 2 & 5 if you wish; see the REM
statement. Simply use PROCclearscreen or PROCclosescreen instead of CLS or
CLG.

 100 DEFPROCclearscreen:LOCAL t%:GCOL0,0
 110 FOR t%=0 TO 512 STEP 4:REM Use STEP 2 in Mode 0
 120 MOVE t%,t%:DRAW t%,1023-t%:DRAW 1279-t%,1023-t%
 130 DRAW 1279-t%,t%:DRAW t%,t%:NEXT:GCOL0,7:ENDPROC

 100 DEFPROCclosescreen:LOCAL t%:GCOL0,0
 110 FOR t%=0 TO 640 STEP 4:REM Use STEP 2 in Mode 0,
     and STEP 8 in Modes 2 & 5
 120 MOVE t%,0:DRAW t%,1023:MOVE 1279-t%,0:DRAW 1279-t%,1023
 130 NEXT:GCOL0,7:ENDPROC