reserve a,b,c for boolean object;
reserve p,q,r,s,A,B,C for Element of LTLB_WFF,
        F,G,X,Y for Subset of LTLB_WFF,
        i,j,k,n for Element of NAT,
        f,f1,f2,g for FinSequence of LTLB_WFF;
reserve M for LTLModel;

theorem Th23:
  M|=A iff M |= {A}
 proof
  thus M|=A implies M |= {A} by TARSKI:def 1;
  A in {A} by TARSKI:def 1;
  hence thesis;
 end;
