Add routine to check the used and free counts.
Avoid memory allocation in trace_and_reclaim_unreachable( ).

Though this doesn't seem to make much of a difference.
Avoid dynamic_cast in reg_is_fully_up_to_date( ).
Add note.
Don't copy the keys map every time we copy a Probability_Model.
Add routine to check the ref_count() of an object.
Make a subA_index_t into an Object.
Share subA indices between data_partitions until we change something.
Stop wasting time on timer_stack routines.
Remove #undef NDEBUG line.
Clean up remove_ownership_mark( ).
Don't perform quadratic out-of-bounds check for every MH move!
Simplify pop_root for only 1 token.