NSHashTable&NSMapTable
本文的單詞摘自NSHiper文件，記錄自己看文件時遇到的生詞。
workhorse
adj. 工作重的；吃苦耐勞的
美 ['wə:khɔ:s]
eg: NSArray are the workhorse collection classes of Foundation.
(這裡可翻譯為主要的)general-purpose
adj. 多用途的；一般用途的
英 ['dʒenərəl'pə:pəs]
eg: use a more general-purpose solution.breaking assumption
n. 打破假設，不符合預期
eg: For NSSet and NSDictionary, the breaking assumption was in the memory behavior when storing objects in the collection.