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 Th19:
  for u,v,w being Element of UN holds {u,v,w} is Element of UN
proof
  let u,v,w be Element of UN;
  {u,v,w} = {u,v} \/ {w} by ENUMSET1:3;
  hence thesis;
end;
