@classmethod
def dual(cls, auxVarPrimals, auxConDuals, options):
    """Convert back the dual value of a constraint that was converted.

        Given a mapping of auxilary variable names (as named in :meth:`convert`)
        to primals and a list of auxilary constraint duals (in the order as the
        constraints were added in :meth:`convert`), returns a dual value for the
        converted constraint.

        :raises NotImplementedError: When dual format not decided upon or not
            known. This will be caught by the reformulation's backward method.
        """
    raise NotImplementedError(
        '{} does not describe how to convert back the dual.'.format(cls.
        __qualname__ if hasattr(cls, '__qualname__') else cls.__name__))
