QueryRoot
   Projection
      ProjectionElemList
         ProjectionElem "s"
         ProjectionElem "p"
         ProjectionElem "o"
      StatementSourcePattern
         Var (name=s)
         Var (name=p)
         Var (name=o)
         StatementSource (id=endpoint1, type=REMOTE)
         StatementSource (id=endpoint2, type=REMOTE)
