# File lib/backports/1.8.7/array/index.rb, line 6
    def index_with_block(*arg)
      return to_enum(:index_with_block) if arg.empty? && !block_given?
      return index_without_block(*arg) unless block_given? && arg.empty?
      each_with_index{|o,i| return i if yield o}
      return nil
    end