Found one of Random Terrains examples ...
http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#sprite_missile_example
x = (missile0x-18)/4 : y = (missile0y)/8 if collision(playfield,missile0) then pfpixel x y off : missile0y = 0 : score = score + 10
I should have figured that out by myself.