Checkpoint, refactoring SL (NB inconsistent, do not checkout).
Checkpoint in SL refactoring, do not use.
Checkpoint, do not use.
Checkpoint, do not use.
Checkpoint, do not use (no more Symheap!)
Checkpoint, do not use (heaps mostly done).
Retired first order prover.
SL prover now works again, others don't.
Improvements to interface conventions, documentation, unification and
subsumption functions for SL. Other provers still non functional.
Tidied up tests directory. Rationalised unification interfaces. Retired
goto prover.
While prover & abducer (mostly) work again.  Time to start work and
merging and fixing bugs!