union Data {
   /* 略 */
    // Queue Interface
    struct Queue {
        union Data* queue;
        union Data* data;
        enum Code whenEmpty;
        enum Code clear;
        enum Code put;
        enum Code take;
        enum Code isEmpty;
        enum Code next;
    } Queue;
    struct SingleLinkedQueue {
        struct Element* top;
        struct Element* last;
    } SingleLinkedQueue;
    struct SynchronizedQueue {
        struct Element* top;
        struct Element* last;
        struct Atomic* atomic;
    } SynchronizedQueue;
   /* 略 */
};