Create a Python function `flatten_nested_list(nested_list)` that takes a list of

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Create a Python function `flatten_nested_list(nested_list)` that takes a list of lists (which may contain other nested lists) and returns a single flattened list containing all the elements in a depth-first order.
Example:
python
Example input:
nested_list = [1, [2, [3, 4], 5], 6, [7, 8, [9, [10]]]]
Expected output:
flattened_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Constraints:
1. The input list can contain integers, other lists, or both.
2. The function should handle arbitrarily deep nesting.
3. You are not allowed to use Python’s built-in `itertools.chain` or similar built-in functions for flattening the list.
Test Cases:
python
Test Case 1:
assert flatten_nested_list([1, 2, [3, 4], [5, [6, 7]]]) == [1, 2, 3, 4, 5, 6, 7]
Test Case 2:
assert flatten_nested_list([[], [1], [[2, 3]], [4, [5, []]]]) == [1, 2, 3, 4, 5]
Test Case 3:
assert flatten_nested_list([[[[1, 2], 3], 4], 5, [], [6, [7, [8, [9]]]]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
Write the `flatten_nested_list` function that passes all the above test cases.

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