add undo functionality

add a function that can pop off the last commmand given to a turtle

ex: `(clean) (forward 10) (undo)` would be functionally equivalent to `(clean)`

No file or test is named. Start by locating the turtle command handling and the representation of the last command, then trace the `(clean) (forward 10) (undo)` example. Done means adding an undo function that removes the last turtle command and makes that example behave like `(clean)`.

Written by the indexing model from the issue text.