staving Alex Fink:
> Well, it's the difference between just inventing the sound changes and
> writing a program to carry them out (and what I want to write is a third
> thing, a program to invent the sound changes).
> Alex
You may find a bit of information theory useful here. Suppose you 
analyse the phonology of a language in terms of distinctive binary 
features, and calculate the probability P of a feature occurring in a 
given context, then the entropy H of the feature in that context is given by

H = -(Plog2(P)+(1-P)log2(1-P))

This is the amount of information carried by that feature in that 
context, measured in bits (in information theory, a bit is the amount of 
information needed to distinguish between two equally likely situations).

The lower the entropy of a feature in a given context, the more likely 
that feature is to undergo change.