{-# LANGUAGE GADTs              #-}
{-# LANGUAGE DeriveDataTypeable #-}

module Backend.Struct where

import Data.Typeable

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-- *
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data Empty a deriving Typeable

data Struct exp a
  where
    Leaf :: Typeable a => exp a -> Struct exp (Empty (exp a))
    Node :: Struct exp a
         -> Struct exp b
         -> Struct exp (a, b)
  deriving
    Typeable

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-- **