code: public static DiskId of(String zone, String disk) {
      return new DiskId(null, zone, disk);
    }
docs: Returns a disk identity given the zone and disk names. The disk name must be 1-63 characters
  long and comply with RFC1035. Specifically, the name must match the regular expression {@code
  [a-z]([-a-z0-9]*[a-z0-9])?} which means the first character must be a lowercase letter, and all
  following characters must be a dash, lowercase letter, or digit, except the last character,
  which cannot be a dash.
  
  @see <a href="https://www.ietf.org/rfc/rfc1035.txt">RFC1035</a>