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diff --git a/src/VeriFuzz/Internal/Circuit.hs b/src/VeriFuzz/Internal/Circuit.hs
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+{-|
+Module      : VeriFuzz.Internal.Circuit
+Description : Internal helpers for generation.
+Copyright   : (c) 2018-2019, Yann Herklotz Grave
+License     : BSD-3
+Maintainer  : ymherklotz [at] gmail [dot] com
+Stability   : experimental
+Portability : POSIX
+
+Internal helpers for generation.
+-}
+
+module VeriFuzz.Internal.Circuit where
+
+import           Data.Graph.Inductive (Graph, Node)
+import qualified Data.Graph.Inductive as G
+import qualified Data.Text            as T
+
+fromNode :: Int -> T.Text
+fromNode node = T.pack $ "w" <> show node
+
+filterGr :: (Graph gr) => gr n e -> (Node -> Bool) -> [Node]
+filterGr graph f = filter f $ G.nodes graph
+
+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
+
+inputs :: (Graph gr) => gr n e -> [Node]
+inputs graph = filterGr graph $ only graph G.indeg G.outdeg
+
+outputs :: (Graph gr) => gr n e -> [Node]
+outputs graph = filterGr graph $ only graph G.outdeg G.indeg