@Override public GroupBy optimize(final QueryContext qc, final VarScope scp) throws QueryException { final int pl = preExpr.length; for (int p = 0; p < pl; p++) { final SeqType it = preExpr[p].seqType(); post[p].refineType(it.withOcc(it.mayBeZero() ? Occ.ZERO_MORE : Occ.ONE_MORE), qc); } return this; }