Whereas hashed and darcs-2 repositories have means for resolving lazy 
patches via _darcs/prefs/sources and caches therein, it should be 
possible for darcs to use these existing sources to search for missing 
patches that are mentioned in context files.

It appears that darcs already attempts this (failing in get_extra), but 
the current context format is lacking a key piece of information 
necessary for patch lookup in a hashed or darcs-2 repository: the patch 
object hash. A simple comparison between a _darcs/hashed_inventory and a 
current context file shows the only key difference between the lack of 
"Hash:" lines.

It would be nice for darcs to support a --hashed-context that contains 
the hash information. It may even be smart to make this the default form 
of context, should it be backwards compatible (in which case the 
existing context may be --old-context), or for the next repository version.

Note that this only gets you as far as the tag that forms the base of the context. 
Beyond that, you also either need the tag to contain hashes or need the context to 
contain the hash of the parent inventory (which is present in the inventory).

I can see how this would be useful for get --context from lazy branches, but I
don't yet have a good idea how beneficial this is going to be.  There is also
something attractive about contexts and inventories being just the same thing.

Two things to note.

First, this won't help for the case where the context file was generated against
a private repository (not that we should expect it to of course)

Second, this change isn't going te be backwards compatible as far as I can tell:

get_context :: B.ByteString -> ([PatchInfo],B.ByteString)
get_context ps =
    case readPatchInfo ps of
    Just (pinfo,r') ->
        case get_context r' of
        (pis,r'') -> (pinfo:pis, r'')
    Nothing -> ([],ps)

I'm a bit uncertain about to deal with the lack of backward compatibility here
and let somebody come up with a plan (need-action).

One approach might just be to support reading them in one version of darcs and
only really start introduce the hashed contexts in a later version.

It might be good to add a simple version scheme to the context files, say
replacing 'Context:' with 'Context-1.x'.  On the other hand, I'm a bit concerned
that the proliferation of version schemes (format, local format [see camp],
patch bundle [issue1099], context)

I'm also marking this Provisional because there is a chance we may have to
decide that the costs (extra complexity, compatibility headaches) are not worth
it.  Basically, this would require somebody really caring about this feature to
work out the details, submit the patch and defend it :-)

Note also that this could come in handy for issue992

It came up that if you want to introduce a new format, then you want to
make sure you have

- format version information
- comments mechanism

Reopening this. Adding the patch hashes as in inventories would be 
useful and make context files more secure.

I was wrong. This makes no sense because patch hashes are not invariant 
under commutation (the patch content can change when commuting patches). 
A bundle with hashes for context patches would be applicable only if the 
receiving repo has all existing patches in the same order as in the 
originating repo.