Monique, I looked in the "Table Alphabetique" and it is not listed there.
It is listed under "anciennes" p. 249, but unless one knew it was medieval, how would one find it?
"Table of Themes," which lists "Anciennes" is on p. 274.

Or does one have to have a gallic mind to use these most peculiar indices?

My eye happened to see "Moby Dick" listed as a folksong (which it isn't) but I couldn't find a page number for it.
(It is an instrumental, also used with a lyric, by Led Zeppelin, but I don't think that is the number referred to).

"Ya se van los pastores", a favorite folksong of mine (esp. as sung by Germaine Montero) is in the "Table Alphabetique" as "Ya se van" but not under the section folksong.

"'Tis a puzzlement."