reserve D,D9 for non empty set;
reserve R for Ring;
reserve G,H,S for non empty ModuleStr over R;
reserve UN for Universe;
reserve R for Ring;
reserve G, H for LeftMod of R;
reserve G1, G2, G3 for LeftMod of R;
reserve f for LModMorphismStr over R;

theorem Th21:
  0 is Element of UN & 1 is Element of UN & 2 is Element of UN
proof
  thus 0 is Element of UN by CLASSES2:56;
  {} is Element of UN & 1 = succ 0 by CLASSES2:56;
  hence
A1: 1 is Element of UN by Th20;
  2 = succ 1;
  hence thesis by A1,Th20;
end;
