(setf (subseq sequence start &optional end) new-subsequence)

start, end - bounding index designators of sequence. The default for end is nil.

subsequence - a proper sequence.

new-subsequence - a proper sequence.

Start specifies an offset into the original sequence and marks the beginning position of the subsequence. end marks the position following the last element of the subsequence.

subseq always allocates a new sequence for a result; it never shares storage with an old sequence. The result subsequence is always of the same type as sequence.

If sequence is a vector, the result is a fresh simple array of rank one that has the same actual array element type as sequence. If sequence is a list, the result is a fresh list.

setf may be used with subseq to destructively replace elements of a subsequence with elements taken from a sequence of new values. If the subsequence and the new sequence are not of equal length, the shorter length determines the number of elements that are replaced. The remaining elements at the end of the longer sequence are not modified in the operation.

 (setq str "012345")  "012345"
 (subseq str 2)  "2345"
 (subseq str 3 5)  "34"
 (setf (subseq str 4) "abc")  "abc"
 str  "0123ab"
 (setf (subseq str 0 2) "A")  "A"
 str  "A123ab"