pre = pres.get(i);
union.addPrecedingOperator(pre);
pre.removeSucceedingOperator(replaceLit);
pre.addSucceedingOperator(new OperatorIDTuple(union, index));
}
union.removePrecedingOperator(replaceLit);
// ReplaceLit becomes the new sucessor of Union
union.setSucceedingOperator(new OperatorIDTuple(replaceLit, 0));
// ReplaceLit gets the joins old sucessors