theorem Th27:
  <*NDSS(V,A)*> IsNDRankSeq V,A
  proof
    set S = <*NDSS(V,A)*>;
    thus S.1 = NDSS(V,A);
    let n be Nat such that
A1: n in dom S & n+1 in dom S;
    dom S = {1} by FINSEQ_1:2,def 8;
    then n = 1 & n+1 = 1 by A1,TARSKI:def 1;
    hence thesis;
  end;
