Remove one level of nesting from arr. Every element that is itself an array/list is replaced by the elements inside it. Anything nested deeper than that stays exactly as it is.
Example 1
Input
arr = [1, [2, 3], [4]]
Output
[1, 2, 3, 4]
Explanation
[2, 3] and [4] are each opened once, and their elements take their place.
Example 2
Input
arr = [[1, [2, 3]], 4]
Output
[1, [2, 3], 4]
Explanation
the outer list is opened, but [2, 3] sits one level deeper, so it stays a list.
Example 3
Input
arr = [1, 2, 3]
Output
[1, 2, 3]
Explanation
no element is a list, so nothing changes.
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