Fooling around with Context Free

have to get to church now, but i was playing with Context Free, and after looking at examples and several tries, i created a checkerboard using it.

the CFDG source is below:

startshape checkerMain

rule checkerMain {
 // modify the numbers at the start to
 // change how many are repeated
 10* { y 2 } checker2 {}
}

rule checker2 {
 10* { x 2 } checker {}
}

rule checker {
 SQUARE {}
 SQUARE { b .5 x 1 }
 SQUARE { b .5 y 1 }
 SQUARE { b 1 x 1 y 1 }
 SQUARE { s .25 b .5 x 1 y 1 }
}