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
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