Write a Python function that takes a list that contains regular elements and other lists and returns a new list that has no sublists (it is "flattened"). Bonus: make your program be able to deal with lists whose sublists also have sublists.

Example: [[1,2], 3, 4] -> [1, 2, 3, 4] Bonus Example: [[[1, 2], 3,], 4, 5, 6]