Best Practices for Sub Repositories

[Curtis Spencer]

Hi,

I am new to mercurial and I am curious about what the best practices  
for external repositories are.

I am currently using subversion and I have a situation like the  
following:

Let foo and bar be internal repos that I control.  Let ext be an  
external repo that may be either an expanded tarball or an external  
subversion/mercurial repo.

/foo
/bar/foo
/bar/ext

In the subversion way, I have foo subdir in bar be an svn:external so  
whenever I update I get the latest changes.  If ext has an open  
source subversion repository, I also make that an svn:external.  Now,  
my question is.  If I move foo and bar to mercurial, what is the best  
practice for keeping the foo subdir in bar synced with the patches  
that go into foo directly.  Also, with respect to ext, which can be  
any type of repository, is a periodic manual merge the way to get the  
changes into the ext subdir of bar?  I see an advantage over  
svn:externals in that updates are not coming in blindly, especially  
if a patch is not backward compatible, but I do not see an easy way  
to manage updates to foo especially if it becomes a subdirectory in  
many projects.

Thanks,
Curtis