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 Th18:
  x in T implies x is FinSequence of NAT
proof
  T c= NAT* by Def3;
  hence thesis by FINSEQ_1:def 11;
end;
