ext = case lex text of
    [(s, r)] → return (s, r)
    _        → []

-- | Surrounds input 'String' with brackets if condition is 'True'.
_surround ∷ Bool → String → String
_surround p s =
_expect ∷ String → ReadS String
_expect s text = case lex text of
    [(s, r)] → return (s, r)
    _        → []

-- | Surrounds input 'String' with brackets if condition is 'True'.
_expect s text = case lex text of
    [(s, r)] → return (s, r)
    _        → []

-- | Surrounds input 'String' with brackets if condition is 'True'.