code: public JSONObject accumulate( String key, Object value ) {
        return _accumulate( key, value, new JsonConfig() );
     }
docs: Accumulate values under a key. It is similar to the element method except
  that if there is already an object stored under the key then a JSONArray
  is stored under the key to hold all of the accumulated values. If there is
  already a JSONArray, then the new value is appended to it. In contrast,
  the replace method replaces the previous value.
  
  @param key A key string.
  @param value An object to be accumulated under the key.
  @return this.
  @throws JSONException If the value is an invalid number or if the key is
  null.