Solve the programming task below in a Python markdown code block.
Write a function that flattens an `Array` of `Array` objects into a flat `Array`.  Your function must only do one level of flattening.

```python
flatten [1,2,3] # => [1,2,3]
flatten [[1,2,3],["a","b","c"],[1,2,3]]  # => [1,2,3,"a","b","c",1,2,3]
flatten [[[1,2,3]]] # => [[1,2,3]]
```
Also feel free to reuse/extend the following starter code:
```python
def flatten(lst):
```