**SUBSEQ** is also **SETF**able, but it won’t extend or shrink a sequence; if the new value and the subsequence to be replaced are different lengths, the shorter of the two determines how many characters are actually changed.

If you need to find a subsequence within a sequence, the function works like **POSITION** except the first argument is a sequence rather than a single item.

It returns **NIL** if the strings match. **MISMATCH** also takes many of the standard keyword arguments: a :key argument for specifying a function to use to extract the values to be compared; a argument to specify the comparison function; and :start1, :end1, :start2, and :end2 arguments to specify subsequences within the two sequences. And a :from-end argument of specifies the sequences should be searched in reverse order, causing **MISMATCH** to return the index, in the first sequence, where whatever common suffix the two sequences share begins.