When we enqueue an atom at level 0, we expect it to stay unconditionally
true whatever happens. However, if it happens that the atom is alrady
true at a level strictly greater than 0 (but lower than base_level),
then enqueue_bool would simply do nothing. To fix this, there is a new
function enqueue_root which modifies in place the level and reason of
the propagated atom if it is already true.
Additionally, enqueue_bool now requires that the atom be undecided,
since if it not (i.e if it is propagated while already decided) it is
most likely a bug.
When a new clause with only 1 atom is added with (usually
from simplifying a theory lemma), the usual strategy is to backtrack to
the base level, then propagate the atom. However, when the atom is
already false at base level, Unsat is raised, in which case the
information that the atom must be true is lost. To avoid that, the
single atom clause is simply pushed back in the stack of clauses to add,
so that it will be re-examined later.
Previously, the heap was not grown when adding local assumptions. This
lead to a bug whne backtracking: indeed when a local assumption was
backtracked, it was added to the (too small) heap, which then raised a
Sparse_vec exception.
Proofs require local assumptions to be recognisable.
Keeping the reason of local assumptions as Bcp simplfies
the code, since a proof is a clause, and allows to not have
to recreate the local unit clause that effectively propagates
the local assumptions.
With this fix, simplification of clauses is not required aymore for
levels between 0 (excluded) and base_level, so the partition function
can be simplified accordingly.
