def index(parent_tree)
      parent_tree = make_object_id(parent_tree)
      if not subtree_of?(parent_tree)
        raise ArgumentError, "#{self.to_s} not a subtree of #{parent_tree.to_s}"
      elsif self.length == parent_tree.length
        raise ArgumentError, "OIDs are the same"
      else
        ObjectId.new(self[parent_tree.length..-1])
      end
    end
def index(parent_tree)
      parent_tree = make_object_id(parent_tree)
      if not subtree_of?(parent_tree)
        raise ArgumentError, "#{self.to_s} not a subtree of #{parent_tree.to_s}"
      elsif self.length == parent_tree.length
        raise ArgumentError, "OIDs are the same"
      else
        ObjectId.new(self[parent_tree.length..-1])
      end
    end
Returns an index based on the difference between this ObjectId
 and the provided parent ObjectId.

 For example, ObjectId.new("1.3.6.1.5").index("1.3.6.1") returns an
 ObjectId of "5".