`SExp` is mutable. So `SExp.true`, `SExp.false`, `SExp.__null__` are not safe to assume it's constant.

```python
from clvm import SExp

print(SExp.true) # This prints '01'

s1 = SExp.to((1, SExp.true))
s1.pair[1].atom = b'\2'

print(SExp.true) # This prints '02'
```

Probably should protect property by getter/setter.