def reverse_comma(s):
    """
    
    Reverse a string with a comma.
    
    Note: ``s`` is expected to be passed through ``cln`` first.
    
    :param s: a string
    :type s: ``str``
    :return: reversed string
    :rtype: ``str``
    """
    if isinstance(s, str) and s.count(',') == 1:
        return ' '.join(s.split(', ')[::-1])
    else:
        return s
