Fix a bug in update and generalize the 'check' predicate to be a simple shorthand to bind the box into something

src/UnitJS/Async/Box.js

@@ -15,15 +15,15 @@ Box.prototype.set = function(newValue) {
 	}.bind(this);
 };
 
-Box.prototype.check = function(predicate) {
-	return function(f) {
-		predicate(this.value)(f);
-	}
+Box.prototype.bind = function(f) {
+	return function(g) {
+		f(this.value)(g);
+	}.bind(this);
 };
 
 Box.prototype.update = function(modifier) {
 	return function(f) {
-		modifier(this.value)(function (newValue) {this.value = newValue; f()});
+		modifier(this.value)(function (newValue) {this.value = newValue; f()}.bind(this));
 	}.bind(this);
 };
 
@@ -36,7 +36,7 @@ function make(initValue) {
 	return {
 		get: box.get.bind(box),
 		set: box.set.bind(box),
-		check: box.check.bind(box),
+		bind: box.bind.bind(box),
 		update: box.update.bind(box),
 	};
 }