Building a new DVector from two DVectors?

Hi there,

I’ve started writing an ordinary differential equation (ODE) propagator/integrator using nalgebra which I plan on using for a high fidelity astrodynamics toolbox. In one representative example of my toolbox, I need to do the following: from a 6 element vector, compute an acceleration, and return a 6-element vector whose top three elements correspond to the bottom three of the input vector and whose bottom three elements correspond to the computed acceleration.

How would I go about doing that?