Optional attributes in instance.

Hi,
I've the following question:

For example, I have an instance like
<person name="Ivan Ivanoff"/> and also "@age" which is the optional 
attribute of a <person> tag.
I need to have an input element which is bound to person/@age and have 
some default value if the attribute is omitted in instance.
My XForms engine (Chiba) ignores my <input> tag unless the @age is 
actually present (it just doesn't show the html input, but shows it's 
label).
Is it possible to do so?

Thank you.

-- 
Alexander Berezhnoy,
Chief Architect
SmartPhone Labs, LLC
http://www.smartphonelabs.com