After reading Greg's ideas on callbacks (which I agree with) I got to thinking, one of the things that would be really useful, is to be able to have a callback to a custom site curve. It would allow user's to have an unprecedented ability to customize a growth model for their particular needs. It would allow for much easier statistical validations of new site curves in the context of the growth model.
There may be other growth functions that would be appropriate to have a customizeable callback, if done properly (none are as clean to implement as a size curve...) -- but one *could* consider the possiblity that when you used a custom callback to a growth function, that the *entire* treelist is passed to the callback, so you'd be free from the worries about passing the right variables. This thought is nebulous at this point, just wanted to throw it "out there" for the collective to contemplate.