Problem Statement

Write a function to flatten the nesting in an arbitrary list of values. Your program should work on the equivalent of this list: Where the correct result would be the list:

Let's start with the solution:

flatten([[[1, 2, 3], 4, [5, [6, 7]], 8], [[9, 10], 11], 12]);
/* [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] */