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 Th27:
  p '&' q <> p & p '&' q <> q
proof
  len p < len(p '&' q) by Th15;
  hence thesis by Th15;
end;
