From 869b054f2b650da23708eefb6cc58e3fb5c1b51d Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Fri, 11 May 2018 12:11:03 -0400
Subject: [PATCH] Define a Monoid instance.

---
 src/Data/Graph.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/Data/Graph.hs b/src/Data/Graph.hs
index efa46a48c..ca1dd6bee 100644
--- a/src/Data/Graph.hs
+++ b/src/Data/Graph.hs
@@ -21,3 +21,7 @@ simplify (Graph graph) = Graph (G.simplify graph)
 
 instance Semigroup (Graph v) where
   (<>) = Class.overlay
+
+instance Monoid (Graph v) where
+  mempty = Class.empty
+  mappend = (<>)