Add "optional" checkbox for explain field's text field

I have a situation where I want to use an explain field with radio button choices, and I want a selection of radio buttons to be required, but I do not wish the explain field text field to be required, even when the trigger value is chosen.

For this, I created a patch to add an "optional" checkbox to the CCK field configuration page, so that the explainfield text field is not required when it's checked.