Thea2 suggested fix - writing ontology/1 at top of plsyn file

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The thea2 library owl2_plsyn.pl writes the ontology/1 declaration at the bottom of a saved .plsyn file, but actually requires it at the top when exporting the file to .ttl format. For example, if you have the following plsyn file:

fu < bar.
ontology(mini).

and try to export it with save_axioms('mini.ttl',ttl). then the exported mini.ttl file will not contain anything other than the ontology declaration (i.e. the fu<bar subClass declaration will be missing).

A possible solution is to ensure (manually) that any ontology/1 declaration is written first. However, this is a problem when going through a cycle of importing from, say, .ttl then saving in .plsyn and then exporting back to .ttl again (presumably after making changes in the plsyn file). The reason is that you would have to remember to move that declaration to the top of the file during editing, because otherwise the export to .ttl won’t work.

A programmatic fix for this, is to modify the owl2_plsyn.pl code so that the ontology/1 declaration is always written at the top of a .plsyn file. The following modification to the write_owl_as_plsyn/1 predicate in that library will accomplish this. I essentially made two changes: firstly, to retrieve the ontology declaration and write it first. Secondly, to prevent any ontology declaration from being written in the main axiom writing forall predicate.

The only slight complication is in the first clause of write_owl_as_plsyn/1. This clause deals with the possibility that multiple ontologies have been loaded – i.e. there are multiple ontology/1 clauses. The clause also looks at the contents of the Opts parameter to see if any ontologies need to be filtered out. I expediently choose the first ontology clause as the name for the new saved ontology. This is a crude choice, but saves having to invent an ontology name. In future, it’s probably better to construct a new ontology name from the file name that is being saved into, but then we still need to decide where to get the URI prefix from.

write_owl_as_plsyn(Opts):-
	setof(Ont,member(ontology(Ont),Opts),Onts),
	!,
	Onts=[FirstOnt|_],		% ensure ontology/1 is written at top...
	writeln(ontology(FirstOnt)),	% use first ontology as new ontology name
	% this clause optimized for ontology filtering
        forall((member(Ont,Onts),
		ontologyAxiom(Ont,A),
		A\=ontology(_),
		\+exclude_axiom(A,Opts)),
	       (   plsyn_owl(Pl,A,Opts),
		   format('~q.~n',[Pl]))).

write_owl_as_plsyn(Opts):-
	% ensure to write ontology/1 at the top, if it exists
	( axiom(ontology(Ont)) -> writeln(ontology(Ont)); true	),
        forall((axiom(A),
		A\=ontology(_),
		\+exclude_axiom(A,Opts)),
	       (   plsyn_owl(Pl,A,Opts),
		   format('~q.~n',[Pl]))).
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