theorem Th28:
  <*NDSS(V,A),NDSS(V,A\/NDSS(V,A))*> IsNDRankSeq V,A
  proof
A1: <*NDSS(V,A)*> IsNDRankSeq V,A by Th27;
    len <*NDSS(V,A)*> = 1 by FINSEQ_1:40;
    hence thesis by A1,Th17;
  end;
