[Conkeror] how to take advantage of form completion?

[Silvio Levy]

Dear Conkeror users,

When I'm typing in a form, Conkeror (like other browsers) often shows
me a menu of earlier things typed in that field, so I can choose one.
I can make the choice using the mouse, but I imagine there is a way
to do it with the keyboard.  I haven't been able to figure it out, however.   

What key(s) should I use in order to select an entry when
conkeror offers completions as I'm typing in a form?

Thanks,

Silvio