How to do hidden fields with SQLFORM.factory?

[Dave S]

I have a factory-built form that I've been happily using for a while, but now I want to add a field that is hidden unless a boolean in that form has been checked.

The right thing for database table form is in the book:

<URL:http://web2py.com/books/default/chapter/29/07/forms-and-validators#Conditional-fields>

Is there an easy way to do this for my non-DB form, or do I have dive into JS libraries?

/dps