allow overriding append.

by putting it back into the Seq type class. the same would be possible
for all those functions defined on Seq, but i don't want to put that
many things in there. later, maybe?

(one way to make all those functions overridable is by making a type
class for each of them and adding a default instance that does what they
currently do. is there an extension for these "extendable" functions?
there probably is.)

hs/DataStructures.hs

    isEmpty  :: s a -> Bool
    -- additional interfaces
    append :: (Seq s) => a -> s a -> s a
    append x s | isEmpty s = cons x nil
    append x s =
        case first s of
             Just x -> cons x $ append x (rest s)
fromList :: (Seq s) => [a] -> s a
fromList [] = nil
fromList (x:xs) = cons x $ fromList xs
        Nothing -> nil
        Just x -> cons x $ take (n-1) (rest s)
append :: (Seq s) => a -> s a -> s a
append x s | isEmpty s = cons x nil
append x s =
    case first s of
        Just x -> cons x $ append x (rest s)
concat :: (Seq s) => s a -> s a -> s a
concat l r | isEmpty l = r
concat l r | isEmpty r = l