This action allows making multiple-action bindings into an atomic
operation that will be undone by a single Undo command.  For example,
to optimize rat lines, you'd delete the rats and re-add them.  To
group these into a single undo, you'd want the deletions and the
additions to have the same undo serial number.  So, you 
An alternative mechanism is freeze/unfreeze for the case restoring between
every two actions is not possible or not practical. The sequence is
Arguments:
| Save | Saves the undo serial number. | 
|---|---|
| Restore | Returns it to the last saved number. | 
| Close | Sets it to 1 greater than the last save. | 
| Block | Does a Restore if there was nothing to undo, else does a Close. | 
| Freeze | Make sure no subsequent actions will bump the undo serial | 
| UnFreeze or Thaw | Allow subsequent actions to bump the undo serial again |