protected static bool requeue(T instance, ref StaticQueue <KEY, T> .Entry state, bool enqueue_if_missing)
 {
     return(!enqueue_if_missing ? StaticQueue <KEY, T> .enqueue(instance, ref state) : StaticQueue <KEY, T> .enrequeue(instance, ref state));
 }
 protected static bool enrequeue(T instance, ref StaticQueue <KEY, T> .Entry state)
 {
     return(!state.inside ? StaticQueue <KEY, T> .enqueue(instance, ref state) : StaticQueue <KEY, T> .requeue(instance, ref state));
 }