This program is amazing - fantastic work. I have been playing with it for a few days now, producing all sorts of things. I quite like the simplicity of this code vs. what it produces.
startshape TREE
rule TREE { BRANCH { b 1 } }
rule BRANCH 20 { DRAW {} BRANCH { y 1 r 3 s 0.98 b -0.015 } }
rule BRANCH {DRAW {} FORK {} }
rule BRANCH{ BRANCH { flip 90 } }
rule FORK { BRANCH { flip 90 } BRANCH { } }
rule DRAW { SQUARE { s 3 1.1 } }
editing the scaling in the DRAW rule (to something as narrow as s 0.3 1.1} can produce some very interesting thin trees, as well. Or playing with the BRANCH rule weighting.
