driver-core/src/main/com/mongodb/internal/connection/ConcurrentLinkedDeque.java
Node<E> back() {
            Node<E> f = predecessor();
            return (f == null || f.isSpecial()) ? null : f;
        }
Node<E> back() {
            Node<E> f = predecessor();
            return (f == null || f.isSpecial()) ? null : f;
        }
Returns previous node containing a nondeleted user element, if
possible.  Use for backward list traversal, but beware that
if this method is called from a deleted node, it might not
be able to determine a usable predecessor.

@return predecessor, or null if no such could be found