Merge pull request #79 from docardoso/demorgan_fix

Fixing De Morgan's laws effect on double negation propositions

Author: pombredanne

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

@@ -522,9 +522,14 @@ def test_demorgan(self):
         algebra = BooleanAlgebra()
         a = algebra.Symbol('a')
         b = algebra.Symbol('b')
+        c = algebra.Symbol('c')
         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((~~(a&b|c)).demorgan(), a&b|c)
+        self.assertEqual((~~~(a&b|c)).demorgan(), ~(a&b)&~c)
+        self.assertEqual(algebra.parse('~'*10 + '(a&b|c)').demorgan(), a&b|c)
+        self.assertEqual(algebra.parse('~'*11 + '(a&b|c)').demorgan(), (~(a&b|c)).demorgan())
 
     def test_order(self):
         algebra = BooleanAlgebra()