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
  {}LTLB_WFF |=0 A implies {}LTLB_WFF |=0 'X' A
proof
  assume Z1: {}LTLB_WFF |=0 A;
A1: {A} |= A by LTLAXIO1:23;
  B in {A} implies {}LTLB_WFF |=0 B by TARSKI:def 1,Z1;
  hence {}LTLB_WFF |=0 'X' A by th265,A1,LTLAXIO1:25;
end;
