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: subset_map.rb
    path of file: ./repos/stackneveroverflow/vendor/bundle/ruby/2.3.0/gems/sass-3.4.24/lib/sass/util
    
    the code of the file until where you have to start completion: require 'set'

module Sass
  module Util
    # A map from sets to values.
    # A value is \{#\[]= set} by p
    
      def []=(set, value)
        raise ArgumentError.new("SubsetMap keys may not be empty.") if set.empty?

        index = @vals.size
        @vals << value
        set.each do |k|
          @hash[k] ||= []
          @hash[k] << [set, set.to_set, index]
        end
      end