Add example

examples/ex1.hs

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+{-# LANGUAGE UnicodeSyntax #-}
+
+import Control.Monad (replicateM)
+import Data.Poset
+import Data.Poset.Morphism
+
+p ∷ Poset Int
+p = fromPairsE [1..7] [(3,1),(3,2),(4,2),(5,3),(5,4),(6,1),(6,4),(7,5),(7,6)]
+
+main ∷ IO ()
+main =
+  mapM_ (print . toPerm) $ filter (nice p) $ generate (length $ elements p)
+
+-- | Generate all possible Morphs over n-size set
+--
+generate ∷ Int → [Morph Int]
+generate n = map fromPerm $ replicateM n [1..n]
+
+-- | Build Morph from permutation
+--
+fromPerm ∷ [Int] → Morph Int
+fromPerm = Morph . zip [1..]
+
+-- | Show Morph as a permutation
+--
+toPerm ∷ Morph Int → [Int]
+toPerm (Morph m) = map snd m