[plt-scheme] Identifying at-exp string literals

Hi all,


I'm looking for a way to write a macro that can distinguish string  
literals that do and don't come from at-expression boilerplate. For  
example, in the following code:

     (define b "b")

     @list{a @b c}

I'd like to be able to differentiate "a " and " c" from "b".


My motivation: I'd like to make the (xml ...) macro from Mirrors.plt  
at-expression-aware so that at-exps and s-exps are ampersand-encoded  
differently. In other words, s-expression "<" and ">" are &-encoded:

     (xml (p "The <br/> tag is a line break."))

          ==> <p>The &lt;br/&gt; tag is a line break.</p>

whereas at-expression "<" and ">" are not:

     @xml{<p>The <br/> tag is a line break.</p>}

          ==> <p>The <br/> tag is a line break.</p>


Any advice would be much appreciated.

Many thanks,

-- Dave

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