def _inv_impl(expr, op, **kw):
    if hasattr(expr, 'negation_clause'):
        return expr.negation_clause
    else:
        return expr._negate()
No fields on a ModelForm should actually result in no fields
dataset/ETHPy150Open zzzeek/sqlalchemy/lib/sqlalchemy/sql/default_comparator.py/_inv_impl--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.4/tests/regressiontests/model_forms_regress/tests.py/EmptyFieldsTestCase.test_empty_fields_on_modelform