javascript - AngularJS dependency-injection -

i have 2 modules:

1. foo.a
2. foo.b

and application module:

  angular.module("foo", ["foo.a","foo.b"]) 

i have service in module foo.b say:

  angular.module("foo.b", [])    angular.module("foo.b").factory("helper",helperfn); 

which want use in 1 of controllers in foo.a.

what have done simple dependency injection:

 angular.module("foo.a", []);   angular.module("foo.a")         .controller("mycontroller",["helper",mycontrollerfn]); 

which working.

my questions

1. how getting "helper" service module foo.b though not declared dependency module a?
2. will break @ later stage?
3. if correct, practice?

put factory need access in both modules in third module. have 2 original modules inject dependency third module.

angular.module("foo", ["foo.a", "foo.b"]);  angular.module("foo.a", ["foo.c"])   .controller("mycontroller", ["helper", mycontrollerfn]);  angular.module("foo.b", ["foo.c"]);  angular.module("foo.c")   .factory("helper", helperfn);