code: def link_to(self, model, idx, self_idx):
          
          if model in self.system.loaded_groups:
  
              # access group instance
              grp = self.system.__dict__[model]
  
              # doing it one by one
              for i, self_i in zip(idx, self_idx):
                  # query model name and access model instance
                  mdl_name = grp._idx_model[i]
                  mdl = self.system.__dict__[mdl_name]
  
                  # query the corresponding uid
                  u = mdl.get_uid(i)
  
                  # update `mdl_from`
                  name_idx_pair = (self._name, self_i)
                  if name_idx_pair not in mdl.mdl_from[u]:
                      mdl.mdl_from[u].append(name_idx_pair)
  
          else:
              # access model instance
              mdl = self.system.__dict__[model]
              uid = mdl.get_uid(idx)
  
              for u, self_i in zip(uid, self_idx):
                  name_idx_pair = (self._name, self_i)
  
                  if name_idx_pair not in mdl.mdl_from[u]:
                      mdl.mdl_from[u].append(name_idx_pair)
docs: Register (self.name, self.idx) in `model._from`
  
          Returns
          -------