Derives Cogen[T] instances for T a singleton type, like
Witness."str".T or Witness.true.T for example.
Derives Cogen[T] instances for T a singleton type, like
Witness."str".T or Witness.true.T for example.
The generated Cogen[T] behaves like Cogen[Unit], as like
Unit, singleton types only have one instance.
Extra contramap, that inserts a next call on the returned seeds,
so that case objects are returned the same Cogen here and when derived through Generic.