docs: def is_valid(self):
          """Returns True if this form and all subforms (if any) are valid.
  
          If all standard form-validation tests pass, uses :class:`~eulxml.xmlmap.XmlObject`
          validation methods to check for schema-validity (if a schema is associated)
          and reporting errors.  Additonal notes:
  
           * schema validation requires that the :class:`~eulxml.xmlmap.XmlObject`
             be initialized with the cleaned form data, so if normal validation
             checks pass, the associated :class:`~eulxml.xmlmap.XmlObject` instance
             will be updated with data via :meth:`update_instance`
           * schema validation errors SHOULD NOT happen in a production system
  
          :rtype: boolean
          """
          valid = super(XmlObjectForm, self).is_valid() and \
                  all(s.is_valid() for s in six.itervalues(self.subforms)) and \
                  all(s.is_valid() for s in six.itervalues(self.formsets))
          # schema validation can only be done after regular validation passes,
          # because xmlobject must be updated with cleaned_data
          if valid and self.instance is not None:
              # update instance required to check schema-validity
              instance = self.update_instance()
              if instance.is_valid():
                  return True
              else:
                  # if not schema-valid, add validation errors to error dictionary
                  # NOTE: not overriding _get_errors because that is used by the built-in validation
                  # append to any existing non-field errors
                  if NON_FIELD_ERRORS not in self._errors:
                      self._errors[NON_FIELD_ERRORS] = self.error_class()
                  self._errors[NON_FIELD_ERRORS].append("There was an unexpected schema validation error.  " +
                      "This should not happen!  Please report the following errors:")
                  for err in instance.validation_errors():
                      self._errors[NON_FIELD_ERRORS].append('VALIDATION ERROR: %s' % err.message)
                  return False
          return valid