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 Th19:
  Seg k, Seg n are_c=-comparable
proof
  n <= k or k <= n;
  then Seg n c= Seg k or Seg k c= Seg n by FINSEQ_1:5;
  hence thesis;
end;
