  Consider the class RBLie of Lie algebras equipped with a Rota---Baxter
operator. Then the forgetful functor RBLie --> Lie has a left adjoint one
denoted by $U_{RB}(\cdot)$. We prove an "operator" analogue of the
Poincare---Birkhoff---Witt theorem for $U_{RB}(L)$, where $L$ is an arbitrary
Lie algebra, by means of Gr\"obner---Shirshov bases theory for Lie algebras
with an additional operator.
