Function member_of_list

fn member_of_list<'ob>(
    elt: Gc<ObjectType<'ob>>,
    list: Gc<ListType<'ob>>,
    eq_fn: for<'ob> fn(_: Gc<ObjectType<'ob>>, _: Gc<ObjectType<'ob>>) -> bool,
) -> Result<Gc<ObjectType<'ob>>>