<control/> with instance mirroring

To communicate between a parent and an embedded XForm via mirroring, the  
@initial attribute is no longer needed, nor the <return/> element; to  
indicate which instance in the parent XForm is to be used, we can provide  
an @instance attribute:

   <control resource="game.xml" instance="values"/>

On the other hand, it ought not need to be a whole instance. Perhaps @ref  
is better:

   <control resource="game.xml" ref="instance('values')"/>
   <control resource="histogram.xml" ref="x[. > 0]"/>

(Or is this asking for trouble?)

The remaining question is how to indicate in the embedded XForm which  
instance is mirrored.

The simplest solution would be to say that the first instance is the  
mirrored one; on the other hand, it is nice to be able to decide yourself  
which the default instance is, to make authoring easier, and your default  
instance always has to be the first.

The @src and @resource attributes on <instance/> usually signify the  
source of the data for an instance, though I am allergic to using special  
schemes for URLs to signify special sources; maybe we could mark an  
instance in some special way... I think it would be a bad idea to specify  
the target instance from the embedding form, since that creates an extra  
dependency between embedder and embeddee: better for the embedded XForm  
specify it.

Steven