reserve
  X,x,y,z for set,
  k,n,m for Nat ,
  f for Function,
  p,q,r for FinSequence of NAT;
reserve p1,p2,p3 for FinSequence;
reserve T,T1 for Tree;

theorem Th21:
  {} in T & <*> NAT in T
  proof
    reconsider x = the Element of T as FinSequence of NAT;
    x in T;
    then
A1: {}^x in T by FINSEQ_1:34;
    {} = <*> NAT;
    hence thesis by A1,Th20;
  end;
