An object containing implicits which are not in the default scope.
Given f, a function from T into S, creates an Ordering[T] whose compare function is equivalent to:
Construct an Ordering[T] given a function
This would conflict with all the nice implicit Orderings
available, but thanks to the magic of prioritized implicits
via subclassing we can make
Ordered[A] => Ordering[A] only
turn up if nothing else works.