code: def _validate_member(self, member, classes, instanceof):
          
          log.info("Validating member %s" % member)
  
          stripped = self._get_stripped_attributes(member, classes)
          if self._field_name_from_uri(member) in stripped:
              all_class_members = sum([self.schema_def.attributes_by_class[cl]
                                       for cl in classes], [])
              if member in all_class_members:
                  return
              if self._namespace_from_uri(member) in self.allowed_namespaces:
                  err = self.err("Unoficially allowed namespace {0}",
                                 self._namespace_from_uri(member))
                  return ValidationWarning(ValidationResult.WARNING, err['err'],
                                           err['line'], err['num'])
          else:
              err = self.err("{0} - invalid member of {1}",
                             self._field_name_from_uri(member),
                             self._field_name_from_uri(instanceof))
              return ValidationWarning(ValidationResult.ERROR, err['err'],
                                       err['line'], err['num'])
docs: return error if `member` is not a member of any class in
          `classes`