public K previousKey(K key) {
        LinkEntry<K, V> entry = (LinkEntry<K, V>) getEntry(key);
        return (entry == null || entry.before == header ? null : entry.before.getKey());
    }
Gets the previous key in sequence.

@param key the key to get before
@return the previous key
jphp-runtime/src/php/runtime/common/collections/map/AbstractLinkedMap.java