Let $G$ be a complex simply-connected semisimple Lie group and let $\g=
\hbox{\rm Lie}\,G$. Let $\g = \n_- +\hh + \n$ be a triangular decomposition of
$\g$. The authors in [LW] introduce a very nice representation theory idea for
the construction of certain elements in $\hbox{\rm cent}\,U(n)$. A key lemma in
[LW] is incorrect but the idea is in fact valid. In our paper here we modify
the construction so as to yield the desired elements in $\hbox{\rm
cent}\,U(\n)$.