code: def add(self, model):
          
          if isinstance(model, list):
              self.algorithms = self.algorithms + model
          else:
              self.algorithms.append(model)
docs: Add a index method.
  
          This method is used to add index algorithms. If multiple algorithms
          are added, the union of the record pairs from the algorithm is taken.
  
          Parameters
          ----------
          model : list, class
              A (list of) index algorithm(s) from
              :mod:`recordlinkage.index`.