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Finally, this exercise shows how to make use of a single list that contains multiple lists and how to use them in a function.

``````list_of_lists = [[1, 2, 3], [4, 5, 6]]

for lst in list_of_lists:
for item in lst:
print item``````
1. In the example above, we first create a list containing two items, each of which is a list of numbers.
2. Then, we iterate through our outer list.
3. For each of the two inner lists (as `lst`), we iterate through the numbers (as `item`) and print them out.

We end up printing out:

``````1
2
3
4
5
6``````

### Instructions

1.

Create a function called `flatten` that takes a single list and concatenates all the sublists that are part of it into a single list.

• On line 3, define a function called `flatten` with one argument called `lists`.
• Make a new, empty list called `results`.
• Iterate through `lists`. Call the looping variable `numbers`.
• Iterate through `numbers`.
• For each number, `.append()` it to `results`.
• Finally, `return results` from your function.