Here, X {\displaystyle X} and Y {\displaystyle Y} may be sets, or they may be types, or they may be other kinds of objects, as explored below.
這裡， X {\displaystyle X} 和 Y {\displaystyle Y} 可以是集合、或者是類型，或者它們可以是其它型別的物件，如下所述。