static sexp
sexp_yuniffi_nccc_proc_release(sexp ctx, sexp self, sexp_sint_t n,
                               sexp procobjptr){
    sexp procobj;
    REQUIRE_TAG(ctx, self, procobjptr, sexp_pointerp, SEXP_CPOINTER);
    procobj = (sexp)sexp_cpointer_value(procobjptr);
    sexp_release_object(ctx, procobj);
    return SEXP_VOID;
}
void callback_free(struct callback* c) {
  ffi_closure_free(c->closure);
  sexp_release_object(c->ctx, c->proc);
}