def detect_line_sep(content):
    """Parse line separator from content.

    >>> detect_line_sep('\\r\\n')
    '\\r\\n'
    >>> detect_line_sep('\\r')
    '\\r'
    >>> detect_line_sep('\\n')
    '\\n'
    >>> detect_line_sep('')
    '\\n'
    """
    if '\r\n' in content:
        return '\r\n'
    elif '\r' in content:
        return '\r'
    else:
        return '\n'
