if (!_.isBoolean(attrDef.required)) {
      throw new Error('Consistency violation: The referenced attribute (`'+attrName+'`, from model `'+modelIdentity+'`) has an unrecognized `required` property in its definition.  By this time, it should always be true or false.  But instead, got: '+util.inspect(attrDef.required, {depth:5})+'');
    }