Sequencing.hs

data Node = Value Int | List [Node] | Sequence [Node]

steps :: Node -> [[Int]]
steps (Value x) = [[x]]
steps (List ns) = case substeps of
    [] -> []
    (x : xs) -> x ++ concat (zipWith prepend lastStates xs)
    where substeps = map steps ns
          lastStates = scanl1 (++) $ map last substeps
          prepend pref ss = map (pref ++) ss
steps (Sequence ns) = concatMap steps ns