ain't的wiki講,originally used for "am not", but also used for "is not", "are not", "has not", or "have not"。
也就是說,ain't等於otherwise。
換句話說這句話的意思是"如果不是以上列舉的情況,那麽。。。。。"
pseudo code as following:
switch(it)
case 1:
statements
break;
case 2:
statements
break;
...
default:
statements
break;