reserve A,B,C,D,p,q,r for Element of LTLB_WFF,
        F,G,X for Subset of LTLB_WFF,
        M for LTLModel,
        i,j,n for Element of NAT,
        f,f1,f2,g for FinSequence of LTLB_WFF;

theorem th14:
  {prop i} |- 'X' prop i & not {prop i} |-0 'X' prop i
proof
  prop i in {prop i} by TARSKI:def 1;then
  {prop i} |- prop i by LTLAXIO1:42;
  hence {prop i} |- 'X' prop i by LTLAXIO1:44;
  thus not {prop i} |-0 'X' prop i by th266,th262ac1;
end;
