static TimeSpan TrialDiv(ulong value)
    {
        var t = new TrialDivision.U64();

        t.IsPrime(value);         // first run.
        var sw = Stopwatch.StartNew();

        if (!t.IsPrime(value))
        {
            throw new Exception("Is not prime.");
        }
        sw.Stop();
        return(sw.Elapsed);
    }
        public static void TrialDivision64Test(ulong value)
        {
            var instance = new TrialDivision.U64();

            Assert.True(instance.IsPrime(value));
        }