The README (https://github.com/logpy/logpy/blob/master/README.md#extending-logpy-to-other-types) seems to make the assumption that the reader knows what unify() does already. (assumes prior knowledge)
A quick introduction (or a link) to what it does and its purpose would help the reader understand that section of the readme better. It would otherwise be better to use another (already introduced) dispatched function.

Also I was looking for a Performance section on the readme to know how expensive it is to do this form of pattern matching.