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 Th23: 'not' TVERUM => p is ctaut
  proof
    let g;
    set v = VAL g;
A1: v.tf = 0 by LTLAXIO1:def 15;
    thus v.('not' TVERUM => p) = v.('not' TVERUM) => v.p by LTLAXIO1:def 15
    .= v.TVERUM => v.tf => v.p by LTLAXIO1:def 15
    .= TRUE => v.tf => v.p by Th4
    .= 1 by A1;
  end;
