theorem Th27: [<*>LTLB_WFF,<*>LTLB_WFF]^ = TVERUM '&&' TVERUM
  proof
    len <*>l = 0;
    then len nega <*>l = 0 by LTLAXIO2:def 4;
    then nega <*>l = 0;
    hence [<*>l,<*>l]^ = TVERUM '&&' TVERUM by LTLAXIO2:10;
  end;
