What is the head table checksum for collections?

It is unclear what the head checksum means for collections.

ULONGcheckSumAdjustmentTo compute: set it to 0, sum the entire font as
ULONG, then store 0xB1B0AFBA - sum.

http://www.microsoft.com/typography/otspec/head.htm

The "entire font" isn't entirely clear for a collection. Does it mean the
whole collection or the whole of the individual font? If the "entire font"
is the individual font within the collection, what happens if the head
table is shared among fonts within the collection?

One option that could make sense would be to define it as over the entire
collection, setting it to 0 for *all* head tables in all fonts in the
collection, doing the sum, and then updating all of them.

FYI, I asked some font experts at Google and the feedback I got was
essentially that this checksum is largely ignored for collections and what
value it should have isn't well defined. If we could get it better defined
that would be great.

Thoughts?