[Radiant] Container tag with default values for nested tags

[Casper Fabricius]

I'm developing a tag inspired by the way <r:navigation /> works; by  
using nested tags to let the user specify how he wants to render a  
given collection.

The user could write:
<r:collection_tag>
	<r:normal><b><r:link></b></r:normal>
</r:collection_tag>

- and the tag uses this nested tag by doing:
     hash = tag.locals.collection_tag = {}
     tag.expand

- and then later:
     hash[:normal].call

- to render the contents of the nested tag.

However, I want the user to also be able to write just:
<r:collection_tag />

- and the tag will populate hash[:normal] with a default value of,  
say, <r:link />.

However, I don't know how to create a default value for a nested tag,  
least not when it contains other Radius tags that need to be parsed in  
the context of each iteration of the collection.

I have tried assigning default values with code such as:
     hash[:normal]   ||= '<li><r:link /></li>'

- or:
     hash[:normal]   ||= Proc.new {'<li><r:link /></li>'}

- but the first is just a string, and does not respond to the #call  
method, and the second is just a proc, and do not in fact render  
<r:link /> in the context, it just returns the string unparsed.

I know of several examples on using default values for tag attributes,  
but not for nested tags. Any hints, ideas, examples?

Cheers,
Casper Fabricius
http://casperfabricius.com