[Petal] Replace contents of tag only if value is set

[Holland, Mark]

> 
> How about this:
> 
>   <p
>     petal:if="true:foo"
>     petal:content="foo"
>   >foo goes here if foo is true</p>
> 
>   <p
>     petal:if="false:foo"
>   >foo wan't true so bar is here</p>
> 
Admittedly this would work, but apart from not tending to very tidy
templates, it still doesn't address my problem that I need to pass the
content's of a tag to a modifier.

Any ideas?

Thanks
Mark