The Runge-Kutta-England tool is a differential equation solver written
in C, which I used a few times in continuous simulation contexts.  This
is a fourth order Runge-Kutta solver combined with an heuristic for
implementing an adaptative step size.

The documentation is kept within file RKE.org, but is more nicely read
by visiting https://pinard.progiciels-bpi.ca/notes/RKE.html with a browser.

François