reserve X,x for set;
reserve k,m,n for Element of NAT,
  p,q,r,s,r9,s9 for Element of HP-WFF,
  T1,T2 for Tree;
reserve T1,T2 for DecoratedTree;
reserve t,t1 for FinSequence;

theorem Th25:
  p => q <> VERUM
proof
  p => q = <*1*>^(p^q) by FINSEQ_1:32;
  then (p => q).1 = 1 by FINSEQ_1:41;
  hence thesis;
end;
