+ /// Return a copy of this TNC.
+ ///
+ /// `M17App` will use this to create a second instance of the supplied TNC then use
+ /// one of them for reading and one of them for writing, concurrently across two threads.
+ ///
+ /// Implementations do not need to worry about trying to make two simultaneous reads or
+ /// two simultaneous writes do something sensible. `M17App` will not do this and it would
+ /// probably produce garbled KISS messages anyway.