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Predicate xref_source_identifier/2
 prolog:xref_source_identifier(+Path, -Id) is semidet[multifile, 'pack_analyzer.pl']
 prolog:xref_source_identifier(+Object, -Ref)[multifile, library(pce_prolog_xref)]
The cross-referencer runs faster if the reference is an indexable term. Therefore we strip the XPCE @ from the object.