reserve A,B,p,q,r,s for Element of LTLB_WFF,
  i,j,k,n for Element of NAT,
  X for Subset of LTLB_WFF,
  f,f1 for FinSequence of LTLB_WFF,
  g for Function of LTLB_WFF,BOOLEAN;

theorem nega <*A*> = <*'not' A*>
   proof
A1:  now
       let n;
       assume that
A2:    1 <= n
       and
A3:    n <= len <*A*>;
       n <= 1 by A3,FINSEQ_1:39;
       then A4: n = 1 by A2,NAT_1:25;
       hence <*'not' A*>.n = 'not' A
       .= 'not' (<*A*>/.n) by FINSEQ_4:16,A4;
     end;
     len <*'not' A*> = 1 by FINSEQ_1:39
     .= len <*A*> by FINSEQ_1:39;
     hence thesis by A1,Def4;
   end;
