Stop unsharing when we record results!
Move version check into invalidate_shared_regs( ).
Stop unsharing computation results.
Fix add_shared_computation( ).
Fix order of mark values in invalidate_shared_regs( ).
Check_used_reg( ): only require some things for root token.
Only the root token has all its re_evaluate regs up to date.
Invalidate values in child_token before removing its parent.
Add member function.
Add more general check.
Don't try to release the parent until the middle of changing the root.
Add comments.
Invalidate shared regs before changing the graph at all.
Add reg_is_shared_with_parent( ).
Simplify find_callers and find_users
Treat regs as modified even if they have no computation in t2.
Share more computations.