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 argument is given, then one plus the index of the rightmost
position in which the sequences differ is returned.