white:=0xff|ff|ff; black:=0;
w:=h:=100; bitmap:=PPM(w,h,white);
x:=w/2; y:=h/2; dir:=0;	// start in middle facing east
do{
   if(bitmap[x,y]){ dir-=1; bitmap[x,y]=black; } // white-->black, turn left
      else        { dir+=1; bitmap[x,y]=white; } // black-->white, turn right
   switch(dir.bitAnd(3)){  // dir is always <0
      case(0){ x+=1; }	// east
      case(1){ y-=1; }	// south
      case(2){ x-=1; }	// west
      case(3){ y+=1; }  // north
   }
}while((0<=x<w) and (0<=y<h));

bitmap.write(File("foo.ppm","wb"));