code: function XUserDefinedEncoder(options) {
      var fatal = options.fatal;
      /**
       * @param {Stream} stream Input stream.
       * @param {number} code_point Next code point read from the stream.
       * @return {(number|!Array.<number>)} Byte(s) to emit.
       */
      this.handler = function(stream, code_point) {
        // 1.If code point is end-of-stream, return finished.
        if (code_point === end_of_stream)
          return finished;
  
        // 2. If code point is an ASCII code point, return a byte whose
        // value is code point.
        if (isASCIICodePoint(code_point))
          return code_point;
  
        // 3. If code point is in the range U+F780 to U+F7FF, return a
        // byte whose value is code point − 0xF780 + 0x80.
        if (inRange(code_point, 0xF780, 0xF7FF))
          return code_point - 0xF780 + 0x80;
  
        // 4. Return error with code point.
        return encoderError(code_point);
      };
    }
docs: 15.5.2 x-user-defined encoder 
  @constructor
  @implements {Encoder}
  @param {{fatal: boolean}} options