dubst_size' :: DUTree s -> ST s Int
dubst_size' t =
    do
    ht <- dubst_bst t
    return (bst_size ht)

-- length of list
dubst_size' :: DUTree s -> ST s Int
dubst_size' t =
    do
    ht <- dubst_bst t
    return (bst_size ht)

-- length of list
dubst_size' t =
    do
    ht <- dubst_bst t
    return (bst_size ht)

-- length of list