/*
  Define type of derivative function.
  The derivative function itself would look like:

int deriv(double x, int ny, double *y, double *dydx, void *pass)
{
    <code>
}
*/
typedef int (*derivf)(double, int, double *, double *, void *);

/* Runge-Kutta integrator */
int rk4(derivf, double, double, int, double *, double *, void *);