Project(
        LeftJoin(
                 Union(
                       BGP(
                           triple(variable(s), uri<http://example.org#p1>, variable(v1))
                       ) ,
                       BGP(
                           triple(variable(s), uri<http://example.org#p2>, variable(v2))
                       )
                 ) ,
                 BGP(
                     triple(variable(s), uri<http://example.org#p3>, variable(v3))
                 )
        ) ,
        Variables([ variable(s), variable(v1), variable(v2), variable(v3) ])
)
