# File lib/bson/grow.rb, line 116
    def b_do!(key, type = BSON::BSON_RUBY::OBJECT) # Append object/array element unfinished
      put(type)
      BSON::BSON_RUBY.serialize_cstr(self, key)
      @b_pos ||= [0]
      @a_index ||= [0]
      @b_pos << @cursor # mark position of size
      @a_index << 0
      put_int(0)
      self
    end