Returns True if no errors are thrown for the specified state.

def is_valid(self, *states):
        """Returns True if no errors are thrown for the specified state."""

        # if no state is specified, fallback to the base FormGroup's is_valid
        if not states:
            return super(StateValidatorFormGroup, self).is_valid()

        # see if the states pass for all forms that define state_validators
        return all(form.is_valid(*states)
                   for form in self.forms
                   if isinstance(form, StateValidatorFormMixin)) and \
               all(self.state_validators[state].is_valid(self)
                   for state in states)