@SuppressWarnings("unchecked")
public static Tuple<Set<Var>> mentionedVarsByPosition(Op... ops) {
Set<Var> graphAcc = collector() ;
Set<Var> subjAcc = collector() ;
Set<Var> predAcc = collector() ;
Set<Var> objAcc = collector() ;
Set<Var> unknownAcc = collector() ;
OpVarsPatternWithPositions visitor = new OpVarsPatternWithPositions(graphAcc, subjAcc, predAcc, objAcc, unknownAcc, false);
for (Op op : ops)
OpWalker.walk(op, visitor);
return Tuple.createTuple(graphAcc, subjAcc, predAcc, objAcc, unknownAcc);
}