They are not needed and complicate the code and make more difficult to
add more parameters to the initialization of Statements.
This produces many changes in the tests. The alternative would be to add
a parameter None in all of them, which are also a lot of changes.
The `Element` class is just used for testing. By using `Element` we are
not testing the actual code. Also, every time we implement a new feature
for the `Feature` class, we need to implement it for `Element` as well.
Replace `Element` by `Integer`.
