private final JSONWriter append(String s) throws JSONException {
        if (s == null) {
            throw new JSONException("Null pointer");
        }
        if (this.mode == 'o' || this.mode == 'a') {
            try {
                if (this.comma && this.mode == 'a') {
                    this.writer.write(',');
                }
                this.writer.write(s);
            } catch (IOException e) {
                throw new JSONException(e);
            }
            if (this.mode == 'o') {
                this.mode = 'k';
            }
            this.comma = true;
            return this;
        }
        throw new JSONException("Value out of sequence.");
    }