XForms: binding to an optional element

I asked this on the XSLTForms list, but it’s worth casting a wider net. Say I have an XML instance like this: <root> <format>xml</format> </root> Possible values for the format element are “xml”, “text”, “binary”, or a default setting, indicated by a complete absence of the <format> element. I’d like this to attach to a select1 with four choices: <xf:select1 ref=”format”>… * [...]