protected QueryIterator execute(OpMinus opMinus, QueryIterator input)
{
Op lhsOp = opMinus.getLeft();
Op rhsOp = opMinus.getRight();
QueryIterator left = executeOp(lhsOp, input) ;
QueryIterator right = executeOp(rhsOp, root()) ;
Set<Var> commonVars = OpVars.patternVars(lhsOp) ;
commonVars.retainAll(OpVars.patternVars(rhsOp)) ;
return new QueryIterMinus(left, right, commonVars, execCxt) ;