Fixed bug where the symbol for the empty heap was not rendering in latex output
Fixed bug where empty heaps were being printed in debug output as true
Modified rules for forming backlinks since the form of weakening implemented for precondition-only sequents is not valid for hoare triples. Also, swapped round the position of equality-based substitution and weakening - I think the way it was before is technically incorrect. Some substantial changes here which almost certainly requires further testing