the idea is that most changes should be undone upon backtracking,
using the global `on_backtrack` command and `at_level_0` to
know when something is going to be permanent.
In particular, should be (possibly optionally) undone on backtracking:
- addition of clauses (clauses being attached)
- propagations of atoms
- addition of literals to the heap
- internalization of literals (tbd)
clauses should also be added immediately, not pushed into a queue
