[Zope] Adding __setitem__ to a formulator field?

there are __getitem__() and get_value() methods for formulator fields. I'd like to experiment with __setitem__ and set_value(). The error message from my naive implementation in field.py is as follows: security.declareProtected('Access contents information', '__setitem__') def __setitem__(self, key, value): self[key] = value self._p_set_changed(1) return value

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app.Formulator.myForm.stringfield1.__setitem__('required',1) Traceback (most recent call last): File "<interactive input>", line 1, in ? AttributeError: __setitem__

Does anyone know what I need to do to add __setitem__ (and later on, set_value()) to formulator fields? Thanks.