theorem Th11: {[<*>LTLB_WFF,<*>LTLB_WFF]}^ = {TVERUM '&&' TVERUM}
 proof
   set Q = [<*>l,<*>l],t = TVERUM;
   hereby
     let x be object;
     assume x in {Q}^;
     then consider P such that
A1:  x = P^ and
A2:  P in {Q};
     P^ = t '&&' t by A2,TARSKI:def 1,LTLAXIO3:27;
     hence x in {t '&&' t} by A1,TARSKI:def 1;
   end;
   let x be object;
   assume x in {t '&&' t};
   then A3: x = t '&&' t by TARSKI:def 1;
   Q in {Q} by TARSKI:def 1;
   hence x in {Q}^ by LTLAXIO3:27,A3;
 end;
