optionalFeatures :: T.Tree (Feature a) -> [Feature a]
optionalFeatures ftree = [fnode f | f <- subtrees ftree, isOptional (fnode f)]
optionalFeatures :: T.Tree (Feature a) -> [Feature a]
optionalFeatures ftree = [fnode f | f <- subtrees ftree, isOptional (fnode f)]
optionalFeatures ftree = [fnode f | f <- subtrees ftree, isOptional (fnode f)]