How can i ensure that an attribute exists?

I tried to use a hidden input field on my charactersheet to store a dummy-variable for the !ammo script because as far as I understood it is not really possible to create a fully fledged dynamic ammunation counter (for fieldset based entries) but the problem is, altough the input field is part of the sheet, the character did not have the attribute yet until I manually added it to the attributes page. Is there a way (by an existing script or macro/etc) to ensure that an attribute exists? When are attributes actually created? On the first input? If so, is there any way to use hidden fields?

You can give the input a default value like value="0" to insure that it gets populated. Or, when you're doing your calculations you can do (@{attribute}+0)+({@{attribute}+0) to keep it from returning a nil value.