Flex :: Actionscript 3 - Detect The Child Element Of A Variable Parent In E4X?

I have XML that looks like this:

<question>
<type_elt>
<opt_out_flag />
</type_elt>
</question>

type_elt is not an element name; it might be <single>, <multiple> or something else, determined at runtime. How, given this, can I detect the presence of the opt_out_flag element?

I tried this (where xml refers to the question element):

if (xml.*.opt_out_flag) {
do_something();
}

but even in cases without opt_out_flag the above expression returns true. Obviously I'm missing something, but what is it?