docs: def check_no_multiple_handlers(self, resource):
          '''The same verb cannot be repeated on several endpoints.'''
          seen = []
          errors = []
          msg = 'HTTP verb "{}" associated to more than one endpoint in "{}".'
          for method in resource.callbacks:
              for op in getattr(method, 'swagger_ops'):
                  if op in seen:
                      errors.append(msg.format(op, resource.__name__))
                  else:
                      seen.append(op)
          return errors