theorem Th4:
  Seg 6 = {1,2,3,4,5,6}
proof
  thus Seg 6 = {1,2,3,4,5} \/ {5 + 1} by Th3,FINSEQ_1:9
    .= {1,2,3,4,5,6} by ENUMSET1:15;
end;
