docs: def refkeys(self,fields):
      "returns {ModelClass:list_of_pkey_tuples}. see syncschema.RefKey. Don't use this yet."
      # todo doc: better explanation of what refkeys are and how fields plays in
      dd=collections.defaultdict(list)
      if any(f not in self.REFKEYS for f in fields): raise ValueError(fields,'not all in',self.REFKEYS.keys())
      for f in fields:
        rk=self.REFKEYS[f]
        for model in rk.refmodels: dd[model].extend(rk.pkeys(self,f))
      return dd