reserve i for Nat, x,y for set;
reserve S for non empty non void ManySortedSign;
reserve X for non-empty ManySortedSet of S;

theorem Th22:
  Seg 9 = {1,2,3,4,5,6,7,8,9}
  proof
    thus Seg 9 = Seg 8 \/ {8+1} by FINSEQ_1:9
    .= {1,2,3,4,5,6,7,8,9} by FINSEQ_3:6,ENUMSET1:84;
  end;
