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 Th34:
  p=>(q=>p) in LTL_axioms
 proof
  p=>(q=>p) is LTL_TAUT_OF_PL
  proof
   let f be Function of LTLB_WFF,BOOLEAN;
   A1: (VAL f).p=0 or(VAL f).p=1 by XBOOLEAN:def 3;
   thus(VAL f).(p=>(q=>p))=(VAL f).p=>(VAL f).(q=>p) by Def15
    .=(VAL f).p=>((VAL f).q=>(VAL f).p) by Def15
    .=1 by A1;
  end;
  hence p=>(q=>p) in LTL_axioms by Def17;
 end;
