'Iterate over the field names with a regular value in cache.'

def __iter__(self):

    (cache, id) = (self._recs.env.cache, self._recs.id)
    dummy = SpecialValue(None)
    for (name, field) in self._recs._fields.iteritems():
        if ((name != 'id') and (not isinstance(cache[field].get(id, dummy), SpecialValue))):
            (yield name)
