simplifyNot :: FeatureExpression -> FeatureExpression
simplifyNot e
    | e == expTrue = expFalse
    | e == expFalse = expTrue
    | otherwise = Not (simplifyExpression e)
simplifyNot :: FeatureExpression -> FeatureExpression
simplifyNot e
    | e == expTrue = expFalse
    | e == expFalse = expTrue
    | otherwise = Not (simplifyExpression e)
simplifyNot e
    | e == expTrue = expFalse
    | e == expFalse = expTrue
    | otherwise = Not (simplifyExpression e)