anchor.diff:      [<Test>]
       member _.TestDerivativesNestedChargedParticle () =
           // 3rd order (fwd-on-fwd-on-rev)
  +        // Nested optimization of a charged particle's trajectory
           // Siskind, J.M., Pearlmutter, B.A. Nesting forward-mode AD in a functional framework. Higher-Order Symb Comput 21, 361–376 (2008). https://doi.org/10.1007/s10990-008-9037-1
           // Page 13
  +        
           let dt = dsharp.tensor(0.1)
           let x0 = dsharp.tensor([0., 8.])
           let x'0 = dsharp.tensor([0.75, 0.])
  
positive.code:         
          Assert.AreEqual(iCorrect, i)
          Assert.True(wfCorrect.allclose(wf, 0.01))
  
negative.code:     [<Test>]
      member _.TestDerivativesNestedPerturbationConfusion () =
          // 2nd order (fwd-on-fwd)
          // Perturbation confusion example
          // Siskind, J.M., Pearlmutter, B.A. Nesting forward-mode AD in a functional framework. Higher-Order Symb Comput 21, 361–376 (2008). https://doi.org/10.1007/s10990-008-9037-1
          // Page 4
  
          let x0 = dsharp.tensor(1)
          let y0 = dsharp.tensor(2)
          let d = dsharp.diff (fun x -> x * dsharp.diff (fun y -> x * y) y0) x0
  