24. use
YUI.prototype.use = function() {
var Y = this, r = [],
args = Array.prototype.slice.call(arguments, 0),
callback = args[args.length - 1],
process = function(names) { ... };
args.pop();
process(args); // calculate sorted full module list r
Y._attach(r); // add the apis to Y
callback(Y);
return this;
};