docs: def add_artifact(self, artifact):
          """Add artifact to :attr:`A`.
  
          :raises TypeError:
              If the artifact is not derived from
              :class:`~creamas.core.artifact.Artifact`.
          """
          if not issubclass(artifact.__class__, Artifact):
              raise TypeError("Artifact to add ({}) is not {}."
                              .format(artifact, Artifact))
          self._A.append(artifact)