jQuery jAM, Season 6

	 
specialForms["set"] = function(args, env){
	if(args.length !=2 || args[0].type != "word")
		throw new SyntaxError("Bad use of set");
	var varName = args[0].name;
	var value = evaluate(args[1], env);
	
	for (var scope = env; scope; scope=Object.getPrototypeOf(scope)){
		if(Object.prototype.hasOwnProperty.call(scope, varName)){
			scope[varName]=value;
			return value;
		}
	}
	throw new ReferenceError("Setting undefined variable " + varName);
};

run("do(define(x,4),",
	" define(setx, fun(val, set(x, val))),",
	" setx(50),",
	" print(x))");
//->50
run("set(quux, true)");
//->Some kind of ReferenceError