an example can be found on AMOSPro_Examples:Examples/H-2/Help_27.AMOS
SETs the mode for drawing a Blitter OBject on screen.
Set Bob number,background,screenplanes,mask
number is the number of the bob you want to affect.
background can take one of three values:
0 replaces the background whenever the bob is moved. (Default)
Greater than 0 replaces the area with a block of colour background-1
Less than 0 doesn't redraw the background at all.
screenplanes is a bit-pattern which specifies which planes are to be used by the bob. A value of 1 draws the bob in plane, a zero doesn't.
mask selects the blitter mode using a set of minterms.