praxix.z5 "multiundo" results in an endless loop

Multi-level undo:
(Note: this capability is not required by the spec.)

Interpreter claims to support undo.

Undo 1 saved...
Undo 2 saved...
Restoring undo 2...
Undo 2 succeeded, return value 2.
loc=77 glob=777
Restoring undo 1...
Undo 2 succeeded, return value 2.
loc=77 glob=777
Restoring undo 1...