It is possible to query the source of a triple (e.g. the URI associated to the document the triple comes from) :
source ?src ?x c:author ?doc
The source can be retrieved and compared in the query :
c:Olivier c:author ?src source ?src ?x c:author ?doc
A set of triples coming from the same source :
source ?src ( ?x c:author ?doc ?doc c:title ?title )
In the previous case, there cannot be an embedded source statement within the parenthesis.
Triples that have no source :
?x ?p ?y source ?src ?x not::?p ?y
Limitation : the source statement does not operate on rdf:type triple where the value is a constant. A triple can have only one source.