## 6.2 Navigation in the proof tree

### 6.2.1 `Undo.`

This command cancels the effect of the last tactic command. Thus, it backtracks one step.
Error messages:

1. `No focused proof (No proof-editing in progress)`
2. `Undo stack would be exhausted`

Variants:

1. `Undo` num`.`
   Repeats `Undo` num times.

### 6.2.2 `Set Undo` num`.`

This command changes the maximum number of `Undo`'s that will be possible when doing a proof. It only affects proofs started after this command, such that if you want to change the current undo limit inside a proof, you should first restart this proof.

### 6.2.3 `Unset Undo.`

This command resets the default number of possible `Undo` commands (which is currently 12).

### 6.2.4 `Restart.`

This command restores the proof editing process to the original goal.
Error messages:

1. `No focused proof to restart`

### 6.2.5 `Focus.`

Will focus the attention on the first subgoal to prove, the remaining subgoals will no more be printed after the application of a tactic. This is useful when there are many current subgoals which clutter your screen.

### 6.2.6 `Unfocus.`

Turns off the focus mode.