code: function _isValid (ndate, nrandom, nchecksum, id) {
    // Guard
    if (!baseURL.isValid(id)) return false;
    if (id.length !== (ndate + nrandom + nchecksum) / 6) return false;
  
    // Test
    var bits = baseURL.decode(id),
      main = bits.slice(0, bits.length - nchecksum),
      check = bits.slice(bits.length - nchecksum, bits.length);
  
    return bitString.checksum(main, nchecksum) === check;
  }
docs: Check if a id is valid
  @private
  @param  {integer} ndate     Size of date bits
  @param  {integer} nrandom   Size of random bits
  @param  {integer} nchecksum Size of checksum bits
  @param  {string}  id        BaseURL encoded Id to test
  @return {Boolean}           [description]