I've extended, simplified, and cleaned up the previous version and separated the examples from the core code. There are now four files;

• pylog.py. This is the core code. 

• logic_variables_examples.py. This illustrates logic variables and how they unify

• train_example.py. This is the original train example. 

• append_example.py. This does append in both directions. That's really something of a parlor trick. In most cases in Prolog one knows which variables should be instantiated on the call and which get instantiated when a function is run. In this case, a single function can go in both directions. That's impressive and startling, but unusual. In my earlier version, there were two Python functions, one for each direction. Now there is only one, which works in both directions.

The four files should be put into a single directory. Each of the example files runs on its own.