AngularJS: reuse component with different parent

Let's say I have A and B reusable components. I want B to invoke a method on A - of course only if B is a child of A. Also - I want B to be used as a standalone component (without A as a parent). In such case the method from non-existing A shouldn't be invoked. My first try was to get controller in link function (exactly in the same way as if I had require: "^A" specified on B - now I cannot have it because I want to use B also as a standalone component) - but this doesn't work:

var module = angular.module("mymodule", []);

// <A>
module.directive("A", function() {
    return {
        restrict: "E",
        transclude: true,
        replace: true,
        scope: {},
        controller: function($scope, $element, $attrs) {
            this.register = function() {
               console.log("Hooray!");
            };
        },
        template:
            '<div class="ng-a" ng-transclude></div>'
    };
});

// <B>
module.directive("B", function() {
    return {
        restrict: "E",
        transclude: true,
        replace: true,
        scope: {},
        link: function($scope, $element, $attrs, refA) {
            if (refA !== undefined) {
                refA.register();
            } else {
                console.log("Standalone");
            }
        },
        controller: function($scope, $element, $attrs) {
            // Do something.
        },
        template:
            '<div class="ng-b" ng-transclude></div>'
    };
});

For use case:

<A><B></B></A><B></B>

console output should be:

Hooray!
Standalone

Any ideas? Thanks!