Fantasy-land: Any interest in adding a `Group` spec?

[evilsoft]

Groups for all?

While it is really strict and probably not grabbed for a lot, I was wondering if it would be beneficial to add a Group spec with the following bits and bobs:

Group

A value that implements the Group specification must also implement
the Monoid specification.

1. g.concat(g.inverse()) is equivalent to g.empty() (right inverse)
2. g.inverse().concat(g) is equivalent to g.empty() (left inverse)

inverse method

inverse :: Group g => g ~> () -> g

[evilsoft]

Actually. Maybe because it is on an instance, it should be more VERB-y and go with invert like Haskell.

[evilsoft]

Well one person is good enough for me!!
Submitted this PR to address this issue.