Fixing De Morgan's laws effect on double negation propositions

Author: Douglas Cardoso

boolean/boolean.py

@@ -1041,7 +1041,7 @@ def demorgan(self):
         This is achieved by canceling double NOTs and using De Morgan laws.
         """
         expr = self.cancel()
-        if expr.isliteral or not isinstance(expr.args[0], (self.NOT, self.AND, self.OR)):
+        if expr.isliteral or not isinstance(expr, self.NOT):
             return expr
         op = expr.args[0]
         return op.dual(*(self.__class__(arg).cancel() for arg in op.args))

boolean/test_boolean.py

@@ -525,6 +525,7 @@ def test_demorgan(self):
         self.assertEqual(algebra.parse('~(a&b)').demorgan(), ~a | ~b)
         self.assertEqual(algebra.parse('~(a|b|c)').demorgan(), algebra.parse('~a&~b&~c'))
         self.assertEqual(algebra.parse('~(~a&b)').demorgan(), a | ~b)
+        self.assertEqual(algebra.parse('~~(a&b|c)').demorgan(), algebra.parse('a&b|c'))
 
     def test_order(self):
         algebra = BooleanAlgebra()