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 Th25: ('not' 'not' p) => p is ctaut
  proof
    let g;
    set v = VAL g;
A1: v.p = 1 or v.p = 0 by XBOOLEAN:def 3;
    thus v.(('not' 'not' p) => p) = v.('not' 'not' p) => v.p by LTLAXIO1:def 15
    .= v.('not' p) => v.tf => v.p by LTLAXIO1:def 15
    .= v.p => v.tf => v.tf => v.p by LTLAXIO1:def 15
    .= 1 by A1, LTLAXIO1:def 15;
  end;
