mismatch

(mismatch sequence1 sequence2 &key test key start1 end1 start2 end2 from-end)

Returns the index into sequence1 of the first position where the two (bounded) sequences differ, or nil when they match element for element. :test compares elements (eql by default) and :key selects the compared value; :start1/:end1/:start2/:end2 bound each sequence. Lite: :from-end is accepted but the scan still runs forward, so the returned index is the forward one.