reserve x,y,z for object, X,Y for set,
  i,k,n for Nat,
  p,q,r,s for FinSequence,
  w for FinSequence of NAT,
  f for Function;

theorem
  tree(elementary_tree 0, elementary_tree 0) = elementary_tree 2
proof
  set T = elementary_tree 0;
  thus tree(T,T) = tree(2 |-> T) by FINSEQ_2:61
    .= elementary_tree 2 by Th54;
end;
