public void ExponentSolveToEulerTest()
 {
     foreach (var sp in Ode.Euler(0,
                                  1,
                                  (t, x) => - x,
                                  new Options {
         RelativeTolerance = 1e-3, InitialStep = 0.01
     }).SolveTo(100))
     {
         Assert.IsTrue(Math.Abs(sp.X[0] - Math.Exp(-sp.T)) < 1e-2);
     }
 }