arrange and pattern matching

-- arrange expression for a _associative_ operation type
-- (therefore ot can be either + or *); examples:
-- (a+(b+c)+d) -> (((a+b)+c)+d), where a, b, c, d are not Add operations
-- (a*(b*c)*d) -> (((a*b)*c)*d), where a, b, c, d are not Mul operations
--    o1                o2
--   / \               / \
--  a  o2      =>     o1 b2
--     / \           / \
--    b1 b2         a  b1
arrange :: Expr -> Expr
-- arr a | trace ("arr " ++ show a) False = undefined
arrange n@Nm{} = n
arrange ex@(Op _o _a@Nm{} _b@Nm{}) = ex
arrange (Op  o  a@Op{}  b@Nm{}) = Op o (arrange a) b
arrange (Op o1 a b@(Op o2 b1 b2)) = case (o1, o2) of
  (Add, Add) -> arrange regroup
  (Add, Sub) -> arrange regroup
  (Mul, Mul) -> arrange regroup
  (Mul, Div) -> arrange regroup
  _          -> Op o1 (arrange a) (arrange b)
  where
    regroup = Op o2 (Op o1 a b1) b2