code: function _passthrough (n, fn) {
    assert(!this._parserCallback, 'There is already a "callback" set!');
    assert(n > 0, 'can only pass through > 0 bytes, got "' + n + '"');
    if (!this._parserInit) init(this);
    debug('passing through %o bytes', n);
    this._parserBytesLeft = n;
    this._parserCallback = fn;
    this._parserState = PASSTHROUGH;
  }
docs: Passes through `n` bytes to the readable side of this stream untouched,
  then invokes `fn` once that amount has been passed through.
  
  @param {Number} n the number of bytes to pass through
  @param {Function} fn callback function to invoke when `n` bytes have passed through
  @api public