what is different between a message "head" and "headers"?

[Matt Swift <swift@alum.mit.edu>]

message.el has `message-narrow-to-head', `message-narrow-to-headers',
and `message-narrow-to-head-or-headers'.


I can read the code, I want to know what the coneptual differnce is.
All this from the manual only seems to suggest that head and headers
are being used in yet one more way than is explained here:


node: terminology

"head"
     The top part of a message, where administrative information (etc.)
     is put.

"header"
     A line from the head of an article.

"headers"
     A collection of such lines, or a collection of heads.  Or even a
     collection of NOV lines.

node: headers

   "Header" is a severely overloaded term.  "Header" is used in RFC1036
to talk about lines in the head of an article (e.g., `From').  It is
used by many people as a synonym for "head"--"the header and the body".
(That should be avoided, in my opinion.)  And Gnus uses a format
internally that it calls "header", which is what I'm talking about
here.  This is a 9-element vector, basically, with each header (ouch)
having one slot.