# File lib/backports/1.8.7/enumerable.rb, line 72
  def find_index(obj = Backports::Undefined)
    if obj != Backports::Undefined
      each_with_index do |element, i|
        return i if element == obj
      end
    elsif block_given?
      each_with_index do |element, i|
        return i if yield element
      end
    else
      return to_enum(:find_index)
    end
    nil
  end