theorem
  n is even implies n,0 are_congruent_mod 4 or n,2 are_congruent_mod 4
  proof
    assume n is even;
    then n mod 4 = 0 or n mod 4 = 2 by Th78;
    then n mod 4 = 0 mod 4 or n mod 4 = 2 mod 4;
    hence thesis by NAT_D:64;
  end;
