docs: def add(self, formula, module, package=None):
          """
          Import module (from package) with formulas, import formulas and add
          them to formula registry.
  
          :param formula: Name of the formula source to add/open.
          :param module: Module containing formula source.
          :param package: [Optional] Package of formula source module.
  
          .. seealso::
              :func:`importlib.import_module`
          """
          super(Formulas, self).add(formula, module, package)
          # only update layer info if it is missing!
          if formula not in self.layer:
              # copy formula source parameters to :attr:`Layer.layer`
              self.layer[formula] = {'module': module, 'package': package}
          self.objects[formula] = self.sources[formula]()
          # register formula and linearity in registry
          formula_src_obj = self.objects[formula]
          meta = [getattr(formula_src_obj, m) for m in self.reg.meta_names]
          self.reg.register(formula_src_obj.formulas, *meta)