Re: n-heads and patch dependency chains

First, hydras or n-head was invented to avoid capping and recapping, and
just advance it as a normal head (and to remember what are
subprojects/patch dependency chains/whatever to choose).

Second, we could generalize those extra commit references in commit
structure (be they "bind", "prior" or "previous", or "depends-on") and have
commit/merge pluggable helper manage them. And merge strategy may make use
of them.

Third, would using *directory* with for a N-HEAD (containing all the
subheads, subprojects, chains, branches, fibers, whatever) instead of an
ordinary file for HEAD be a good idea? For hydra if we want it to be easily
interweaved with ordinary commit I think we would also need the link for
bottom, hydra shoulder, hydra tail i.e. common commit being starting point
for all the chains, or subprojects (for subprojects it can be empty tree
commit).

-- 
Jakub Narebski
Warsaw, Poland