You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: base.rb
    path of file: ./repos/bullet/lib/bullet/registry
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Bullet
  module Registry
    class Base
      attr_r
    
      def add(key, value)
        @registry[key] ||= Set.new
        if value.is_a? Array
          @registry[key] += value
        else
          @registry[key] << value
        end