pub fn contains(&self, a: &T, b: &T) -> bool {
match (self.index(a), self.index(b)) {
(Some(a), Some(b)) => self.with_closure(|closure| closure.contains(a.0, b.0)),
pub fn contains(&self, a: &T, b: &T) -> bool {
match (self.index(a), self.index(b)) {
(Some(a), Some(b)) => self.with_closure(|closure| closure.contains(a.0, b.0)),