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{-# LANGUAGE FlexibleInstances, TypeSynonymInstances, UndecidableInstances #-}
module Tc173a where
class FormValue value where
isFormValue :: value -> ()
isFormValue _ = ()
class FormTextField value
instance FormTextField String
instance {-# OVERLAPPABLE #-} FormTextField value => FormTextFieldIO value
class FormTextFieldIO value
instance FormTextFieldIO value => FormValue value
instance {-# OVERLAPPING #-} FormTextFieldIO value => FormTextFieldIO (Maybe value)
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