stable-sort

(stable-sort sequence predicate &key key)

Sorts sequence like sort, but preserves the relative order of elements that predicate considers equal (neither (predicate a b) nor (predicate b a) is true). The optional :key function is applied to each element before comparison. Unlike Common Lisp's destructive stable-sort, the result is always a fresh list — the argument is not modified, and a string or vector argument also comes back as a list of its elements.