Scieneer Common Lisp 1.3.9 online documentation

mismatch sequence1 sequence2 &key from-end test test-not start1 end1 start2 end2 key[Function]

The specified subsequences of sequence1 and sequence2 are compared element-wise. If they are of equal length and match in every element, the result is nil. Otherwise, the result is a non-negative integer, the index within sequence1 of the leftmost position at which they fail to match; or, if one is shorter than and a matching prefix of the other, the index within sequence1 beyond the last position tested is returned. If a non-'nil :from-end keyword argument is given, then one plus the index of the rightmost position in which the sequences differ is returned.