Refactor to only release leaf tokens.
Move destroy_all_computations_in_context( ) into release_child_token( ).
Move swapping of mappings out of merge_split_mapping( ).
Separate swap_tokens( ) and pivoting the root token.
Use swap_tokens( ) instead of actualy swapping the mappings.
Start tracking source tokens in merge_split_mapping( ).
Make pivot_mapping change the source_token for computations it moves.
Assert that source tokens are correct!
Add a missed assert.