Add a comment for flattenSCC

Author: meooow25

containers/src/Data/Graph.hs

@@ -266,7 +266,8 @@ flattenSCCs = concatMap flattenSCC
 -- 'flattenSCC1' has the more precise type.
 flattenSCC :: SCC vertex -> [vertex]
 flattenSCC (AcyclicSCC v) = [v]
-flattenSCC (NECyclicSCC vs) = NE.toList vs
+flattenSCC (NECyclicSCC (v :| vs)) = v : vs
+-- Note: Best to avoid NE.toList, it is too lazy.
 
 -- | The vertices of a strongly connected component.
 --