code: def convert(self, plugin=None):
          
          if plugin:
              plugin = kurt.plugin.Kurt.get_plugin(plugin)
              if plugin.name in self._plugins:
                  return self._plugins[plugin.name]
              else:
                  err = BlockNotSupported("%s doesn't have %r" %
                          (plugin.display_name, self))
                  err.block_type = self
                  raise err
          else:
              return self.conversions[0]
docs: Return a :class:`PluginBlockType` for the given plugin name.
  
          If plugin is ``None``, return the first registered plugin.