@@can.onKeysRemoved( handler(newValue) )

The @@can.onKeysRemoved symbol points to a function that registers handler to be called back when the keys on the object change via properties deleted.

var obj = {
    handlers: {},
    removeKeyValue: function(key){
        var result = delete this[key];
        var keyStillExists = this[canSymbol.for("hasOwnKey")](key);
        var self = this;
        if(result && !keyStillExists) {
            obj.handlers.__keysRemoved.forEach(function(handler){
                handler.call(self, [key]);
            });
        }
    }
};

obj[canSymbol.for("can.onKeysRemoved")] = function(handler){
    if(!obj.handlers.__keysRemoved) {
        obj.handlers.__keysRemoved = [];
    }
    obj.handlers.__keysRemoved.push(handler);
}