  An abtract $\nu$-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 additional concrete special instance of the abstract $\nu$-metric, by
verifying all the assumptions demanded in the abstract set-up. This example
links the abstract $\nu$-metric with the one proposed by Vinnicombe as a
candidate for the $\nu$-metric for nonrational plants.
