@abc.abstractmethod<EOL><INDENT>def is_valid(self):<DEDENT>
return<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: ``false`` if there is a known error in this form, ``true`` otherwise
        :rtype: ``boolean``
        :raise: ``OperationFailed`` -- attempt to perform validation failed

        *compliance: mandatory -- This method must be implemented.*