revVariance :: Variance -> Variance
revVariance v = case v of
    InVar -> NonVar
    CoVar -> ContraVar
    ContraVar -> CoVar
    NonVar -> InVar

-- | compute the minimal variance
revVariance :: Variance -> Variance
revVariance v = case v of
    InVar -> NonVar
    CoVar -> ContraVar
    ContraVar -> CoVar
    NonVar -> InVar

-- | compute the minimal variance
revVariance v = case v of
    InVar -> NonVar
    CoVar -> ContraVar
    ContraVar -> CoVar
    NonVar -> InVar

-- | compute the minimal variance