monoid as category
幺半群可以认为是只有一个对象（A）的范畴，此时，态射必须是A→A的。
范畴对于identity arrow和结合律的要求和幺半群 monoid一致。此时，在monoid的定义中的集合实际上就是范畴的态射集。
One object categories (monoids) are like groups without inverses. Categories with many objects have the added complication that one cannot always compose one arrow with another.