reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r for Real;
reserve p,p1,p2 for Prime;

theorem Th50:
  21|^2 + 1 = 2*13*17
  proof
    21|^2 = 21*21 by WSIERP_1:1;
    hence thesis;
  end;
