SCREEN 12
clr1 = 6
clr2 = 7
FOR y = 20 TO 370 STEP 50
        FOR x = 20 TO 370 STEP 50
                LINE (x, y)-(x + 50, y + 50), clr1, BF
                SWAP clr1, clr2
        NEXT x
        SWAP clr1, clr2
NEXT y