What is HEAD?

[Ming Kin Lai]

Appendix A-5 of the 1.12.12 version cederqvist says "HEAD refers to the most 
recent version  available in the repository" when it describes "-r tag".  
This is the only place in cerderqvist that has a formal definition of HEAD.
Q1: Are there some versions in the repository that are not available?  
Actually I think the use of the word "version" here violates the convention 
stipulated in Section 4.2: "To avoid confusion, the word version is almost 
never used in this document."  (That's not true at all, there are many uses 
of the word 'version'.)  If I replace the word version with revision, then 
the definition will become "HEAD refers to the most recent revision 
available in the repository" - well, once a change is committed as a 
revision, then it is always available, isn't it?  If you are talking about 
uncomitted "version", then it will not be in the repository, so the word 
'available' seems to be redundant and misleading, does it?
Q2: What does it mean by "the most recent"?  In terms of the time the 
revision is committed?Suppose I committed revision 1.2 today, and revision 
1.1.2.1 on a branch yesterday, so 1.2 is the most recent version and is thus 
HEAD?
Q3: Is HEAD dependent on where my working copy is?  That is, is it true that 
if my working copy is on the trunk (i.e. it is based on a revision on the 
trunk), then HEAD refers to the most recent revision the trunk, but if my 
working copy is on a branch (i.e. it is based on a revision on the branch), 
then HEAD refers to the most recent revision on the branch?  See also 
questions below.
Appendix A-8 describes "annotate" and says "For each file in files, print 
the head revision of the trunk, ..."
Q4: is this "head revision of the trunk" the same as HEAD?  So, is there a 
"head revision" of each branch?  And I assume that "annotate" will not print 
the head revision of a branch by default even if my working copy is based on 
that branch.
Appendix A-15 sort of defines "the head revision" as "the latest revision on 
the trunk".  It implies that there is one and only one "head revision" - 
that's why it speaks of _the_ head revision.  Well, but A-8 implies there 
can be many head revisions - one is of the trunk, and also one for each 
branch!
Q5: If there are many head revisions, then does HEAD depend on where my 
working copy is?
Appendix A-17 appears to suggest that HEAD depends on where my working copy 
is, beacuse it speaks of "the current revisions".
Q6: Specifically, suppose I tag revision 1.2 as Root-of-mybranch:
cvs tag Root-of-mybranch
before creating a branch:
cvs tag -b mybranch
And then I make changes and commit them as revision 1.3 (on the trunk).
Now I switch to the branch:
cvs update -r mybranch
and now I am on revision 1.2.  OK, but does this "update -r mybranch" 
effectively switch me to a branch?
If it does, and assumed that HEAD depends on where my working copy is, that 
is, HEAD now should be 1.2, then
cvs diff -r HEAD -r 1.2 file
should output nothing.
If it does not, that is, I am still on the trunk, then HEAD should be 1.3, 
and
cvs diff -r HEAD -r 1.2 file
should output the difference between 1.2 and 1.3.
According to my experiment, it output the difference between 1.2 and 1.3.  
So, does or does not
cvs update -r mybranch
put me on the branch?
Now, given that I am already on the branch (I think), if I make changes and 
commit, the new revision will be 1.2.2.1.
If now I
cvs diff -r HEAD -r 1.2.2.1 file
does CVS output nothing as HEAD now is just 1.2.2.1 or outout the difference 
between 1.3 and 1.2.2.1?
Would the situation be any difference if
cvs checkout -r mybranch
?

Incidentally, can anyone be kind enough to clarify A.19.1 of the manual 
about "update -r tag[:date]"?  The way it is written suggests that there are 
two ways to supply valid argumnets to tag and date:
1. give only the value of tag and that value is a revision, e.g. 1.2
2. give both tag and date where tag is a branch tag such as "mybranch".
What happens if I give only tag and tag is a branch tag?  (same problem with 
the documnetation about checkout)

(By the way, is a Table of Contents deliberately omitted from cederqvist?)

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