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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def service_set_tag(path, service_name, tag):
    '''
    Change the tag of a docker-compose service
    This does not pull or up the service
    This wil re-write your yaml file. Comments will be lost. Indentation is set to 2 spaces

    path
        Path where the docker-compose file is stored on the server
    service_name
        Name of the service to remove
    tag
        Name of the tag (often used as version) that the service image should have

    CLI Example:

    .. code-block:: bash

        salt myminion dockercompose.service_create /path/where/docker-compose/stored service_name tag
    '''
    compose_result, err = __load_docker_compose(path)
    if err:
        return err
    services = compose_result['compose_content']['services']
    if service_name not in services:
        return __standardize_result(False,
                                    'Service {0} did not exists'.format(service_name),
                                    None, None)
    if 'image' not in services[service_name]:
        return __standardize_result(False,
                                    'Service {0} did not contain the variable "image"'.format(service_name),
                                    None, None)
    image = services[service_name]['image'].split(':')[0]
    services[service_name]['image'] = '{0}:{1}'.format(image, tag)
    return __dump_compose_file(path, compose_result,
                               'Service {0} is set to tag "{1}"'.format(service_name, tag),
                               already_existed=True)