code: def services_type(loader):
      
  
      def impl(string):
          t_services = loader.get_models('common.models#Services')
          if set(string) - set("bqtf"):
              raise ValueError
          return t_services(_str=''.join(set(string)))
  
      return impl
docs: Returns a function which validates that services string contains only a combination of blob, queue, table,
      and file. Their shorthand representations are b, q, t, and f.