theorem Th5:
  Seg 7 = {1,2,3,4,5,6,7}
proof
  thus Seg 7 = {1,2,3,4,5,6} \/ {6 + 1} by Th4,FINSEQ_1:9
    .= {1,2,3,4,5,6,7} by ENUMSET1:21;
end;
