Conditional Attributes?

Hi,

I'm trying to use conditional attribute values to use the correct image formats when producing HTML or PDF; I want PNG images included for HTML, and SVG images included for PDF.

According to the AsciiDoc docs here: http://www.methods.co.nz/asciidoc/chunked/ch28.html

Attribute references contained in the entry <value> will be expanded.

I'm probably doing something incorrectly, but I can't get this to work with AsciiDoctor. Given this document:

= Attribute Test
:foo: lish
:bar: {foo}
:qaz: wsx
:edc: {qaz@wsx:enabled:disabled}

Testing output of {foo} and {bar}.

Testing output of {qaz} and {edc}.

The (lightly trimmed) output is:

<div class="paragraph">
<p>Testing output of lish and lish.</p>
</div>
<div class="paragraph">
<p>Testing output of wsx and {qaz@wsx:enabled:disabled}.</p>
</div>

Shouldn't the last paragraph read:

<p>Testing output of wsx and enabled.</p>

Thanks!