RangedSet.patch

diff --git a/Data/Ranged/RangedSet.hs b/Data/Ranged/RangedSet.hs
index eca86ef..915db37 100644
--- a/Data/Ranged/RangedSet.hs
+++ b/Data/Ranged/RangedSet.hs
@@ -72,8 +72,10 @@ infixl 5 -<=-, -<-, -?-
 newtype DiscreteOrdered v => RSet v = RSet {rSetRanges :: [Range v]}
    deriving (Eq, Show)
 
+instance DiscreteOrdered a => Semigroup (RSet a) where
+    (<>) = rSetUnion
+
 instance DiscreteOrdered a => Monoid (RSet a) where
-    mappend = rSetUnion
     mempty = rSetEmpty
 
 -- | Determine if the ranges in the list are both in order and non-overlapping.