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Predicate read_nquad/2
 read_nquad(+Stream, -Quad) is det
Read the next quad from Stream as Quad. Stream must have UTF-8 encoding.
Quad- is a term quad(Subject,Predicate,Object,Graph). Arguments follow the normal conventions of the RDF libraries. NodeID elements are mapped to node(Id). If end-of-file is reached, Quad is unified with end_of_file.
- syntax_error(Message) on syntax errors