# File lib/bson/grow.rb, line 94
    def push_doc!(bson) # Appends BSON doc with correct key unfinished
      @a_index ||= [0]
      @b_pos ||= [0]
      put(BSON::BSON_RUBY::OBJECT)
      put_binary(@a_index[-1].to_s)
      put(0)
      @a_index[-1] += 1
      put_binary(bson.to_s)
      self
    end