Definition (RDF Quadruple). Let S be a set of data sources, which is a subset of the set of IRIs, i.e., S ⊂ ?. An ordered pair (t,g) of a triple t := (s,p,o) and a graph g ∈ S and t ∈ get(c) is a triple in graph g, where get is a HTTP get request, and the result of the request cannot be an empty set, i.e., get(s) ≠ Ø. The quadruple (s,p,o,g) is called an RDF quadruple (RDF quad).