string-/=, etc.

[DCP@QUABBIN.SCRC.Symbolics.COM]

Page 301 is slightly ambiguous.  If you read it carefully, some parts
imply it is a relative index ("...then the result is the index within
the strings..." and "put another way, the result is the length of the
logest common prefix...") but other parts imply it is absolute ("The
index returned in case of a mistmatch is an index into /string1/.")  The
last sentence on the page is the real clincher, but is said much too
late and the almost-relative wording.

[In hindsight, some of us may think the answer should be relative to the
beginning of the specified substrings, not absolute in string1, but
that's incompatible and probably too incompatible.]