There should be a test for the string-split-lens case where the input violates the lens laws

[jackfirth]

Additionally the [else (error 'bad)] line is unnecessary.

[AlexKnauth]

I think that either the else case should be there (possibly with a better error like "this should never happen"), or we should use a version of cond that raises an error instead of returning void (like in the teaching languages, I think). In case it becomes more complicated later and we forget something.

[jackfirth]

A cond/strict could be added to the utils