def write_lists_as_separate_lines(lists, output_file):
    """
Write lists to a text file on separate lines. For example,
the lists ["1", "2", "3"] and ["another", "list"] are
written to the file as:
1 2 3
another list

Parameters
----------
lists: a list of lists
  A `list` of lists that are written to a file.
output_file: str
  The name of the output file to which the lists are written.

Returns
-------
None
  The function only writes lists to a text file, nothing is returned.
    """
    assert type(lists) == type([])
    # Make sure lists is of type 'list'

    with open(output_file, "w") as write_file:
        for l in lists:
            assert type(l) == type([])
            # Make sure the elements in lists are also lists

            write_file.write("%s\n" % " ".join(map(str, l)))

    return None