DECLARE FUNCTION mo! (x!, y!)
SCREEN 13
FOR i = 0 TO 63: PALETTE i, i: PALETTE i + 64, 256 * i + i: NEXT
rr = 90: xxx = 1: yyy = -1: zzz = 1.5
k = 1'.22
gg = rr * SQR(xxx * xxx + yyy * yyy + zzz * zzz)
FOR yy = -rr TO rr
  FOR x = -rr * 1.534954 TO rr * 1.534954 STEP .8333
      xx = SIN(x / rr) * SQR(rr * rr - yy * yy)
      zz = SQR(ABS(rr * rr - xx * xx - yy * yy))
      c = (xxx * xx + yyy * yy + zzz * zz) / (gg)
     IF c > 0 THEN
       IF ((mo(x, 16)) - 8) * ((mo(yy, 16)) - 8) >= 0 THEN
       PSET (160 + xx * k, 100 - yy), c * 63
       ELSE
        PSET (160 + xx * k, 100 - yy), c * 63 + 64
       END IF
     END IF
   NEXT
NEXT

FUNCTION mo (x, y)
  r = x MOD y
  IF r < 0 THEN r = r + y
  mo = r
END FUNCTION