def get_single_value_from_xref_dic(xref_dic):
    """
    If the given dictionary contains only one entry, get its value,
    independent of key.

    Args:
        xref_dic (dict): A nested dictionary of the form {'xref':
                                                            {xref:
                                                            {'value': value,
                                                             'desc': desc}
                                                            }
                                                         }
    Returns:
        value (str): The value of 'value' the innermost dict
    """
    if xref_dic:
        if len(xref_dic['xref'].keys()) > 1:
            return None
        else:
            k = next(iter(xref_dic['xref']))
            value = xref_dic['xref'][k]['value']
    else:
        value = '-'
    return value