eval :: FeatureConfiguration a -> FeatureExpression -> Bool
eval config (FeatureRef f) = elem f [fId x | x <- flatten (fcTree config)]
eval :: FeatureConfiguration a -> FeatureExpression -> Bool
eval config (FeatureRef f) = elem f [fId x | x <- flatten (fcTree config)]
eval config (FeatureRef f) = elem f [fId x | x <- flatten (fcTree config)]