Function pre_eval_on_exit 

fn pre_eval_on_exit(
    args: &[Arg],
    env: &Environment,
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Register an expression to be evaluated when the current function exits.

@param expr expression to evaluate on exit (or NULL to clear) @param add if TRUE, append to existing on.exit expressions; if FALSE, replace them @param after if TRUE (default), append after existing; if FALSE, prepend before existing @return NULL, invisibly