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+-- | Internal structure of the DirectedGraph type. Not exported in the library.
+--
+module Hakyll.Core.DirectedGraph.Internal
+    ( Node (..)
+    , DirectedGraph (..)
+    ) where
+
+import Prelude hiding (reverse, filter)
+import Data.Monoid (Monoid, mempty, mappend)
+import Data.Set (Set)
+import Data.Map (Map)
+import qualified Data.Map as M
+import qualified Data.Set as S
+
+-- | A node in the directed graph
+--
+data Node a = Node
+    { nodeTag        :: a        -- ^ Tag identifying the node
+    , nodeNeighbours :: (Set a)  -- ^ Edges starting at this node
+    } deriving (Show)
+
+-- | Append two nodes. Useful for joining graphs.
+--
+appendNodes :: Ord a => Node a -> Node a -> Node a
+appendNodes (Node t1 n1) (Node t2 n2)
+    | t1 /= t2 = error "appendNodes: Appending differently tagged nodes"
+    | otherwise = Node t1 (n1 `S.union` n2)
+
+-- | Type used to represent a directed graph
+--
+newtype DirectedGraph a = DirectedGraph {unDirectedGraph :: Map a (Node a)}
+                        deriving (Show)
+
+-- | Allow users to concatenate different graphs
+--
+instance Ord a => Monoid (DirectedGraph a) where
+    mempty = DirectedGraph M.empty
+    mappend (DirectedGraph m1) (DirectedGraph m2) = DirectedGraph $
+        M.unionWith appendNodes m1 m2