Add new member children_of_token( ).
Rename split_reg to unshare_and_clear_result( ).
Don't record any changeable operations if not evaluate_changeable.
Make it so that token 0 is never used.
Remove unused variable.
Use t==0 to indicate that changeables are in normal form in incremental_eval.
Remove unneeded token parameter.
Stop using token arg to new_modifiable.
For dot_graph_for_token( ).
Remove token argument to new_modifiable.
Install dirtiness condition for incremental_evaluate( ).
Change return type to void.
Install condition for cleanness in set_reg_value( ).