The `source/gen-*.pl` files are created by `partial_reduce_thun/0`.  I'm
just messing around with it at the moment.  In theory the reduced forms
would be more efficient, but there's no pressure to improve performance
yet, and I'm not e.g. feeding the output to GNU Prolog to compile to
machine code, eh?

It's just neat to see what the reducer makes of it.