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 for k be Nat st k in dom f holds
  f/.k => 'not' ((con nega f)/.(len con nega f)) is ctaut
  proof
    let k be Nat;
    assume
A1: k in dom f;
    set q = f/.k, p = q => alt(f);
    assume
    not p is ctaut;
    then consider g such that
A2: not (VAL g).p = 1;
    set v = VAL g;
    v.p = 0 by A2,XBOOLEAN:def 3;
    then A3: v.q => v.alt(f) = 0 by LTLAXIO1:def 15;
    v.alt(f) = TRUE or v.alt(f) = FALSE by XBOOLEAN:def 3;
    hence contradiction by A3,Th20,A1;
  end;
