  An abstract $\nu$-metric was introduced by Ball and Sasane, with a view
towards extending the classical $\nu$-metric of Vinnicombe from the case of
rational transfer functions to more general nonrational transfer function
classes of infinite-dimensional linear control systems. In this short note, we
give an important concrete special instance of the abstract $\nu$-metric, by
verifying that all the assumptions demanded in the abstract set-up are
satisfied when the ring of stable transfer functions is the Hardy algebra
$H^\infty$. This settles the open question implicit in \cite{BalSas2}.
