def is_valid(self):
<EOL>return True<EOL>
Tests if ths form is in a valid state for submission.

        A form is valid if all required data has been supplied compliant
        with any constraints.

        return: (boolean) - ``false`` if there is a known error in this
                form, ``true`` otherwise
        raise:  OperationFailed - attempt to perform validation failed
        *compliance: mandatory -- This method must be implemented.*