Make MdToken a Semigroup and a Monoid, so that I can use 'mempty' for

it.

src/MdToHTML.hs

@@ -60,6 +60,12 @@ instance Show MdToken where
   show (Strikethrough token) = "<s>" ++ show token ++ "</s>"
   show (Unit unit) = printf "%s" unit
 
+instance Semigroup MdToken where
+  a <> b = Document [a, b]
+
+instance Monoid MdToken where
+  mempty = Unit ""
+
 -- ---------------
 -- Helpers
 leftmostLongest :: (Foldable t) => [(a, t b)] -> (a, t b)