Adding (not rewriting a class) for the form generated by form_for helper

Question

How do you add an extra class to the form formed by the rails? This is in contrast to overwriting an existing class, as described here:

How do you redefine the class name in form_for helper?

Usage: class => "foo" does not work.

Answer 1

Following the advice on the comments, I did this by manually adding the source class