How to access a non-child controller?

a.js

$.a.addEventListener("click", function(e) {
  // What goes here?
});

a.tss

".container": {
  backgroundColor: "red",
  height: "50%"
}

a.xml

<Alloy>
  <View class="container" id="a">
  </View>
</Alloy>

b.js

$.b.addEventListener("click", function(e) {
  $.b.hide();
});

b.tss

".container": {
  backgroundColor: "blue",
  height: "50%"
}

b.xml

<Alloy>
  <View class="container" id="b">
  </View>
</Alloy>

index.tss

".container": {
  backgroundColor:"white",
  layout: "vertical"
}

index.xml

<Alloy>
  <Window class="container">
    <Require src="a" id="a"/>
    <Require src="b" id="b"/>
  </Window>
</Alloy>