extend network protocol to stop clients from locking servers now all repositories have capabilities slot, tuple with list of names. if 'unbundle' capability present, repo supports push where client does not need to lock server. repository classes that have unbundle capability also have unbundle method. implemented for ssh now, will be base for push over http. unbundle protocol acts this way. server tells client what heads it has during normal negotiate step. client starts unbundle by repeat server's heads back to it. if server has new heads, abort immediately. otherwise, transfer changes to server. once data transferred, server locks and checks heads again. if heads same, changes can be added. else someone else added heads, and server aborts. if client wants to force server to add heads, sends special heads list of 'force'.

# basic operation
adding a
reverting a
changeset 2:b38a34ddfd9f backs out changeset 1:a820f4f40a57
a
# file that was removed is recreated
adding a
adding a
changeset 2:44cd84c7349a backs out changeset 1:76862dcce372
content
# backout of backout is as if nothing happened
removing a
changeset 3:0dd8a0ed5e99 backs out changeset 2:44cd84c7349a
cat: a: No such file or directory
# backout with merge
adding a
reverting a
changeset 3:6c77ecc28460 backs out changeset 1:314f55b1bf23
merging with changeset 2:b66ea5b77abb
merging a
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
line 1