code: @SuppressWarnings("unused") // called through reflection by RequestServer
    public FramesV3 columnDomain(int version, FramesV3 s) {
      Frame frame = getFromDKV("key", s.frame_id.key());
      Vec vec = frame.vec(s.column);
      if (vec == null)
        throw new H2OColumnNotFoundArgumentException("column", s.frame_id.toString(), s.column);
      s.domain = new String[1][];
      s.domain[0] = vec.domain();
      return s;
    }
docs: TODO: return VecDomainV4