ekmett/semigroups — pull request #20 by nandykins (2013-09-27 19:43:39+00:00)
Title: Add instance Semigroup a => Semigroup (Const a b)

I'm also planning on getting ‘instance Monoid a => Monoid (Const a b)’ into base (for lens), so this might as well be provided for symmetry.