-- a.k.a join
-- >>> let t1 = tip (bin (tip (tip 1), tip (bin (tip 2, tip 3))))
-- >>> t1
-- Tip (Bin (Tip (Tip 1)) (Tip (Bin (Tip 2) (Tip 3))))
-- >>> mu t1
-- Bin (Tip (Tip 1)) (Tip (Bin (Tip 2) (Tip 3)))
-- >>> mu . mu $ t1
-- Bin (Tip 1) (Bin (Tip 2) (Tip 3))
-- >>> fmap mu t1
-- Tip (Bin (Tip 1) (Bin (Tip 2) (Tip 3)))
-- >>> mu . fmap mu $ t1
-- Bin (Tip 1) (Bin (Tip 2) (Tip 3))
mu = {- (| id, alpha . inr |) = (| id, [tip, bin] . inr |) = -} foldt (id, bin)
mu = {- (| id, alpha . inr |) = (| id, [tip, bin] . inr |) = -} foldt (id, bin)
mu = {- (| id, alpha . inr |) = (| id, [tip, bin] . inr |) = -} foldt (id, bin)