code: function pack(arr, buf) {
    assert(Array.isArray(arr), 'array expected');
    var i;
    buf = buf || new Buffer(0);
    buf = _preamble(buf, arr);
    for(i = 0;i < arr.length;i++) {
      buf = _arr(buf, arr[i]);
    }
    return buf;
  }
docs: Pack an array of arrays.
  
  Typcially used when a caller wishes to pack multiple command arrays.
  
  @param arr An array of arrays.
  @param buf A buffer to append the encoded contents to (optional).
  
  @return A buffer with the RESP encoded contents.