merge

(merge result-type sequence-1 sequence-2 predicate &key key)

Merges two already-sorted sequences into one sorted sequence of result-type. predicate is the same ordering predicate sort takes, and :key selects what it sees. The merge is stable: when neither element precedes the other, the one from sequence-1 comes first.

result-type may be a run-time value, but it is limited to the sequence families coerce builds -- list, vector and string. Unlike Common Lisp's, this merge does not destroy its arguments.