code: def validate(self):
          
          if not isinstance(self.location, Location):
              raise TypeError(u'Expected Location location, got: {} {}'.format(
                  type(self.location).__name__, self.location))
  
          if self.location.field:
              raise ValueError(u'Expected location to point to a vertex, '
                               u'but found a field: {}'.format(self.location))
docs: Validate that the ContextFieldExistence is correctly representable.