'Notify that fields have been modified on ``self``. This invalidates
the cache, and prepares the recomputation of stored function fields
(new-style fields only).
:param fnames: iterable of field names that have been modified on
records ``self``'

@api.multi
def modified(self, fnames):

    spec = []
    for fname in fnames:
        spec += self._fields[fname].modified(self)
    self.env.invalidate(spec)
