context-free art

A little piece of context-free art:

circles.cfdg

startshape trails
background {b -1}
rule trail {
trail{
sat 0.001
a -0.001
r 0.4
y 0.5
s 0.999y 0.5
}
SQUARE{s 1}
}
rule trail 0.001 {
trail{flip 90} ring{x 5}
}
rule trails {
140*{r 120}trail{b 1 sat 0 h 100}
}
rule ring {
120*{r 5}SQUARE{s 4 0.5 y 6 h 100}
}