[Gsll-devel] How to pass a null pointer to apply-step

Hello, I am solving two coupled ODEs, one with respect to time, another to z. An element in the solution of the first ODE is used in the second one. So I have to use the lower-level apply-step function: (apply-step stepper time y step-size yerr dydt-in dydt-out) The gsl manual says dydt-in can be a null pointer. I wonder how to use null pointer in apply-step? Thank you. Leo -- H A P P Y H O L I D A Y S!