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diff --git a/src/Verismith/Circuit/Internal.hs b/src/Verismith/Circuit/Internal.hs
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+{-|
+Module      : Verismith.Circuit.Internal
+Description : Internal helpers for generation.
+Copyright   : (c) 2018-2019, Yann Herklotz
+License     : BSD-3
+Maintainer  : yann [at] yannherklotz [dot] com
+Stability   : experimental
+Portability : POSIX
+
+Internal helpers for generation.
+-}
+
+module Verismith.Circuit.Internal
+    ( fromNode
+    , filterGr
+    , only
+    , inputs
+    , outputs
+    )
+where
+
+import           Data.Graph.Inductive (Graph, Node)
+import qualified Data.Graph.Inductive as G
+import qualified Data.Text            as T
+
+-- | Convert an integer into a label.
+--
+-- >>> fromNode 5
+-- "w5"
+fromNode :: Int -> T.Text
+fromNode node = T.pack $ "w" <> show node
+
+-- | General function which runs 'filter' over a graph.
+filterGr :: (Graph gr) => gr n e -> (Node -> Bool) -> [Node]
+filterGr graph f = filter f $ G.nodes graph
+
+-- | Takes two functions that return an 'Int', and compares there results to 0
+-- and not 0 respectively. This result is returned.
+only
+    :: (Graph gr)
+    => gr n e
+    -> (gr n e -> Node -> Int)
+    -> (gr n e -> Node -> Int)
+    -> Node
+    -> Bool
+only graph fun1 fun2 n = fun1 graph n == 0 && fun2 graph n /= 0
+
+-- | Returns all the input nodes to a graph, which means nodes that do not have
+-- an input themselves.
+inputs :: (Graph gr) => gr n e -> [Node]
+inputs graph = filterGr graph $ only graph G.indeg G.outdeg
+
+-- | Returns all the output nodes to a graph, similar to the 'inputs' function.
+outputs :: (Graph gr) => gr n e -> [Node]
+outputs graph = filterGr graph $ only graph G.outdeg G.indeg