【计】 链中唯一单元

only only middling read-only only too only just read only only to only that if only only not only child element only-in-chain(OIC) only element only element of chain only-element-of-chain IF-AND-ONLY-IF element if-and-only-if element middle-in-chain (MIC) first-in-chain (FIC) first-in-chain last-in-chain(LIC) first-in-chain(FIC)