[RFC] branch --bind

[Eric Siegerman]

On Fri, 2010-01-08 at 18:36 +1100, Robert Collins wrote: 
> In the following conditions:
> B bound to A, B.parent == A
> C bound to A
> 
> You should always use 'update' - it knows how to preserve the left
hand
> status of A's parents in the working tree when history has forked.
> 
> Pulling in a bound branch where B.parent != B.master works well, of
> course - I wasn't talking about that case though.
> 

I'm having trouble understanding this, on a number of levels:
 1. Are you simply saying that, in a bound branch, one must never
    "pull" from that branch's master, but should always "update"
    instead?
    
    In other words, is it the actual source branch of the "pull"
    that matters, rather than the bound branch's configured
    "parent" value?  Or does B.parent==A somehow trigger the
    problem even if one explicitly does "cd B; bzr pull
    SomeOtherBranchEntirely"?

 2. What does branch C have to do with it?

 3. If one does pull inappropriately, what goes wrong?  You
    allude to what happens to the data structures, but what does
    it mean to me as an end user?

Thanks,
  - Eric