theorem
  for V being LeftMod of R, W1,W2 being Submodule of V holds
  W1 + W2 = the ModuleStr of V
  iff for v being Vector of V ex v1, v2 being Vector of V
  st v1 in W1 & v2 in W2 & v = v1 + v2 by Lm17;
