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

def was_modified_since(header=None, mtime=0, size=0):
    '''Check if an item was modified since the user last downloaded it

    :param header: the value of the ``If-Modified-Since`` header.
        If this is ``None``, simply return ``True``
    :param mtime: the modification time of the item in question.
    :param size: the size of the item.
    '''
    header_mtime = modified_since(header, size)
    if header_mtime and header_mtime <= mtime:
        return False
    return True