<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def collect_garbage():
    '''
    Completely removed all currently 'uninstalled' packages in the nix store.

    Tells the user how many store paths were removed and how much space was freed.

    :return: How much space was freed and how many derivations were removed
    :rtype: str

    .. warning::
       This is a destructive action on the nix store.

    .. code-block:: bash

        salt '*' nix.collect_garbage
    '''
    cmd = _nix_collect_garbage()
    cmd.append('--delete-old')

    out = _run(cmd)

    return out['stdout'].splitlines()