use default arg in included file

edit

Say I have a number of launch files with arguments, like so:

//master.launch
<launch>
 <arg name="filename">
 <arg name="world" default="?????">
 <include file="${find package}/launch/$(arg filename).launch">
  <arg name="world" value="$(arg world)">
 </include>
</launch>

//child1.launch
<launch>
 <arg name="world" default="world1">
 <!-- do stuff -->
</launch>

//child2.launch
<launch>
 <arg name="world" default="world2">
 <!-- do stuff -->
</launch>

I want to set the world arg in master so that if I don't specify it, it will take the default in the child files.

However, the child defaults within each file are different, as you can see (world1 and world2).

How can i achieve this by replacing ????? ?