`(probability (PROBABILITY BODY ...) ...)`syntaxSimilar to a

`cond`form, but evaluates a clause based on a given probability.`PROBABILITY`is expected to be a number less than 1.0, or the symbol`else`. The sum of all probabilities should add up to 1.0, or less than 1.0 if an`else`clause is used. Otherwise there is a chance that no clauses will be evaluated (in the case that the probabilities add to less than 1.0 and no`else`is used) or a clause may never be evaluated (in the case that the probabilities add to more than 1.0).For example:

(probability (0.1 '((branch (flower)) (apex))) (0.7 #f) (else '((branch (flower)) (branch (flower)) (apex)))

describes a 10% chance of creating a branch with a flower, a 70% chance of doing nothing, and a 20% chance of creating two branches with flowers.