reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

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;
