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```from prefect import Flow, flatten


files = preprocess_csv.map(flatten(list_of_csvs))```
used to be this ^

```@task
def flatten(nested: list[list]) -&gt; list:
    return [el for arr in nested for el in arr]```

```nested = [list(range(5)) for x in range(5)]

nested
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

sum(nested, [])
[0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4]```