Set the value of the (proxy) field

def set(self, instance, value, **kw):
        """Set the value of the (proxy) field
        """
        proxy_field = self.get_proxy_field(instance)
        if proxy_field is None:
            return None
        # set the field with the proper field manager of the proxy field
        fieldmanager = IFieldManager(proxy_field)
        return fieldmanager.set(instance, value, **kw)