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 Th4: (VAL g).TVERUM = 1
  proof
    thus (VAL g).TVERUM = (VAL g).TFALSUM => (VAL g).TFALSUM by LTLAXIO1:def 15
    .= FALSE => (VAL g).TFALSUM by LTLAXIO1:def 15
    .= 1;
  end;
