- open_source(+File, -Stream) is semidet[multifile, library(prolog_clause)]
 - Hook into clause_info/5 that opens the stream holding the source
for a specific clause. Thus, the query must succeed. The default
implementation calls open/3 on the File property.
clause_property(ClauseRef, file(File)), prolog_clause:open_source(File, Stream)
 - prolog_clause:open_source(+File, -Stream) is semidet[multifile, library(swish/trace)]
 - Open SWISH non-file sources.