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 Th1:
  Seg 3 = {1,2,3}
proof
  thus Seg 3 = {1,2} \/ {2 + 1} by FINSEQ_1:2,9
    .= {1,2,3} by ENUMSET1:3;
end;
