theorem Th6:
  for x being set, T1, T2 holds (x-tree (T1,T2)).{} = x
proof
  let x be set, T1, T2;
  x-tree (T1,T2) = x-tree<*T1,T2*> by TREES_4:def 6;
  hence thesis by TREES_4:def 4;
end;
