source' :: [MediaType] -> Maybe SourceDer
source' (x:xs) = case x of
    (Source a) -> Just a
    _ -> source' xs
source' :: [MediaType] -> Maybe SourceDer
source' (x:xs) = case x of
    (Source a) -> Just a
    _ -> source' xs
source' (x:xs) = case x of
    (Source a) -> Just a
    _ -> source' xs