theorem Th17:
  g + (LG1+LG2) = (g+LG1) + (g+LG2)
  proof
    now let h;
      thus(g+(LG1+LG2)).h = (LG1+LG2).(h-g) by Def1
                         .= LG1.(h-g)+LG2.(h-g) by RLVECT_2:def 10
                         .=(g+LG1).h+LG2.(h-g) by Def1
                         .=(g+LG1).h+(g+LG2).h by Def1
                         .=((g+LG1)+(g+LG2)).h by RLVECT_2:def 10;
    end;
    hence thesis;
  end;
