/**
* Marks key as unlocked.
*
* @param key Key.
* @return {@code True} if last locked key was unlocked.
*/
boolean unlocked(Object key) {
locked.remove(key);
return locked.isEmpty();
}
/**
* Marks key as locked.
*
* @param key Key.
*/
void locked(Object key) {
locked.add(key);
}