# File lib/haml/util/subset_map.rb, line 47
      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