customize-face + inherit

[Yuri D'Elia]

I'd like to customize a face so that it inherits *all* values from it's
parent. That is, I want the face to be an exact copy of the parent.

To do that, I can add the 'inherit' property to the desired parent face,
but I would also need to *clear* the rest of the attributes from the
face being customized, such as foreground.

How can I do that conveniently from the customize-face interface?
(how can I 'nil' a value?).

Thanks