<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_swagger_versions(settings):
 """ Validates and returns the versions of the Swagger Spec that this pyramid application supports. :type settings: dict :return: list of strings. eg ['1.2', '2.0'] :raises: ValueError when an unsupported Swagger version is encountered. """

    swagger_versions = set(aslist(settings.get(
        'pyramid_swagger.swagger_versions', DEFAULT_SWAGGER_VERSIONS)))

    if len(swagger_versions) == 0:
        raise ValueError('pyramid_swagger.swagger_versions is empty')

    for swagger_version in swagger_versions:
        if swagger_version not in SUPPORTED_SWAGGER_VERSIONS:
            raise ValueError('Swagger version {0} is not supported.'
                             .format(swagger_version))
    return swagger_versions