theorem Th4:
  PRs(F) c= [:[:C_3(F),C_3(F):],[:C_3(F),C_3(F):]:]
proof
  let x be object;
  4C_3(F) = [:[:C_3(F),C_3(F):],[:C_3(F),C_3(F):]:];
  hence thesis by Def8;
end;
