DeMorgan not applied in a specific case

In some situations DeMorgan rule is not being applied.

Here is a sample

```python
import boolean
algebra = boolean.BooleanAlgebra()
TRUE, FALSE, NOT, AND, OR, symbol = algebra.definition()

x = NOT(AND(NOT(symbol("x0")), NOT(symbol("x4"))))
print(x.simplify())
```

The actual output is 
```
~(~x0&~x4)
```

But one would expect
```
x0|x4
```