code: def _add_conversion(self, plugin, pbt):
          
          assert self.shape == pbt.shape
          assert len(self.inserts) == len(pbt.inserts)
          for (i, o) in zip(self.inserts, pbt.inserts):
              assert i.shape == o.shape
              assert i.kind == o.kind
              assert i.unevaluated == o.unevaluated
          if plugin not in self._plugins:
              self._plugins[plugin] = pbt
docs: Add a new PluginBlockType conversion.
  
          If the plugin already exists, do nothing.