theorem Th40:
  (L1 - L2).v = L1.v - L2.v
proof
  thus (L1 - L2).v = L1.v + (- L2).v by Th22
    .= L1.v + (- L2.v) by Th36
    .= L1.v - L2.v by RLVECT_1:def 11;
end;
