@@can.offKeyValue( key, handler(newValue) )

the @@can.offKeyValue symbol is placed on a Map-like object to point to a function used to stop listening to changes on the key property with the handler function.

var obj = {
    handlers: {},
    setKeyValue: function(key, value){
        this[key] = value;
        var self = this;
        obj.handlers[key].forEach(function(handler){
            handler.call(self, value);
        });
    }
};

obj[canSymbol.for("can.offKeyValue")] = function(key, handler){
    if(!obj.handlers[key]) {
        obj.handlers[key] = [];
    }
    obj.handlers[key].splice(obj.handlers[key].indexOf(handler), 1);
}