docs: def add(self, output, module, package=None):
          """
          Add output to
          """
          super(Outputs, self).add(output, module, package)
          # only update layer info if it is missing!
          if output not in self.layer:
              # copy output source parameters to :attr:`Layer.layer`
              self.layer[output] = {'module': module, 'package': package}
          # instantiate the output object
          self.objects[output] = self.sources[output]()
          # register outputs and meta-data in registry
          out_src_obj = self.objects[output]
          meta = [getattr(out_src_obj, m) for m in self.reg.meta_names]
          self.reg.register(out_src_obj.outputs, *meta)