ya existe una derivación para la misma función.
checkRedef :: (Decl d) => d -> DerivDecl -> EDeriv' ()
checkRedef d d' = when (sameDecl d d') $ Left ([RedefinedDerivation $ getNameDecl d'],d')

-- | Funcion que 
checkRedef :: (Decl d) => d -> DerivDecl -> EDeriv' ()
checkRedef d d' = when (sameDecl d d') $ Left ([RedefinedDerivation $ getNameDecl d'],d')
checkRedef d d' = when (sameDecl d d') $ Left ([RedefinedDerivation $ getNameDecl d'],d')