The biggest difference is that your stores (interval 1 N) as an N-element list. Theirs stores it as an object with 2 slots, while still being a subtype of SEQU

The biggest difference is that your stores (interval 1 N) as an N-element list. Theirs stores it as an object with 2 slots, while still being a subtype of SEQUENCE so many built-in functions work as expected.