theorem Th39:
  for R being comRing for V being RightMod of R for L1, L2 being
  Linear_Combination of V holds L1 + L2 = L2 + L1
proof
  let R be comRing;
  let V be RightMod of R;
  let L1, L2 be Linear_Combination of V;
  let v be Vector of V;
  thus (L1 + L2).v = L2.v + L1.v by Def9
    .= (L2 + L1).v by Def9;
end;
