rebase: support --detach when null is common ancestor

hgext/rebase.py

@@ -1,577 +1,577 b''
 # rebase.py - rebasing feature for mercurial
 #
 # Copyright 2008 Stefano Tortarolo <stefano.tortarolo at gmail dot com>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 '''command to move sets of revisions to a different ancestor
 
 This extension lets you rebase changesets in an existing Mercurial
 repository.
 
 For more information:
 http://mercurial.selenic.com/wiki/RebaseExtension
 '''
 
 from mercurial import hg, util, repair, merge, cmdutil, commands
 from mercurial import extensions, ancestor, copies, patch
 from mercurial.commands import templateopts
 from mercurial.node import nullrev
 from mercurial.lock import release
 from mercurial.i18n import _
 import os, errno
 
 nullmerge = -2
 
 def rebase(ui, repo, **opts):
     """move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of
     history (the source) onto another (the destination). This can be
     useful for linearizing *local* changes relative to a master
     development tree.
 
     You should not rebase changesets that have already been shared
     with others. Doing so will force everybody else to perform the
     same rebase or they will end up with duplicated changesets after
     pulling in your rebased changesets.
 
     If you don't specify a destination changeset (``-d/--dest``),
     rebase uses the tipmost head of the current named branch as the
     destination. (The destination changeset is not modified by
     rebasing, but new changesets are added as its descendants.)
 
     You can specify which changesets to rebase in two ways: as a
     "source" changeset or as a "base" changeset. Both are shorthand
     for a topologically related set of changesets (the "source
     branch"). If you specify source (``-s/--source``), rebase will
     rebase that changeset and all of its descendants onto dest. If you
     specify base (``-b/--base``), rebase will select ancestors of base
     back to but not including the common ancestor with dest. Thus,
     ``-b`` is less precise but more convenient than ``-s``: you can
     specify any changeset in the source branch, and rebase will select
     the whole branch. If you specify neither ``-s`` nor ``-b``, rebase
     uses the parent of the working directory as the base.
 
     By default, rebase recreates the changesets in the source branch
     as descendants of dest and then destroys the originals. Use
     ``--keep`` to preserve the original source changesets. Some
     changesets in the source branch (e.g. merges from the destination
     branch) may be dropped if they no longer contribute any change.
 
     One result of the rules for selecting the destination changeset
     and source branch is that, unlike ``merge``, rebase will do
     nothing if you are at the latest (tipmost) head of a named branch
     with two heads. You need to explicitly specify source and/or
     destination (or ``update`` to the other head, if it's the head of
     the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
     Returns 0 on success, 1 if nothing to rebase.
     """
     originalwd = target = None
     external = nullrev
     state = {}
     skipped = set()
     targetancestors = set()
 
     lock = wlock = None
     try:
         lock = repo.lock()
         wlock = repo.wlock()
 
         # Validate input and define rebasing points
         destf = opts.get('dest', None)
         srcf = opts.get('source', None)
         basef = opts.get('base', None)
         contf = opts.get('continue')
         abortf = opts.get('abort')
         collapsef = opts.get('collapse', False)
         extrafn = opts.get('extrafn')
         keepf = opts.get('keep', False)
         keepbranchesf = opts.get('keepbranches', False)
         detachf = opts.get('detach', False)
         # keepopen is not meant for use on the command line, but by
         # other extensions
         keepopen = opts.get('keepopen', False)
 
         if contf or abortf:
             if contf and abortf:
                 raise util.Abort(_('cannot use both abort and continue'))
             if collapsef:
                 raise util.Abort(
                     _('cannot use collapse with continue or abort'))
             if detachf:
                 raise util.Abort(_('cannot use detach with continue or abort'))
             if srcf or basef or destf:
                 raise util.Abort(
                     _('abort and continue do not allow specifying revisions'))
 
             (originalwd, target, state, skipped, collapsef, keepf,
                                 keepbranchesf, external) = restorestatus(repo)
             if abortf:
                 return abort(repo, originalwd, target, state)
         else:
             if srcf and basef:
                 raise util.Abort(_('cannot specify both a '
                                    'revision and a base'))
             if detachf:
                 if not srcf:
                     raise util.Abort(
                         _('detach requires a revision to be specified'))
                 if basef:
                     raise util.Abort(_('cannot specify a base with detach'))
 
             cmdutil.bail_if_changed(repo)
             result = buildstate(repo, destf, srcf, basef, detachf)
             if not result:
                 # Empty state built, nothing to rebase
                 ui.status(_('nothing to rebase\n'))
                 return 1
             else:
                 originalwd, target, state = result
                 if collapsef:
                     targetancestors = set(repo.changelog.ancestors(target))
                     external = checkexternal(repo, state, targetancestors)
 
         if keepbranchesf:
             if extrafn:
                 raise util.Abort(_('cannot use both keepbranches and extrafn'))
             def extrafn(ctx, extra):
                 extra['branch'] = ctx.branch()
 
         # Rebase
         if not targetancestors:
             targetancestors = set(repo.changelog.ancestors(target))
             targetancestors.add(target)
 
         sortedstate = sorted(state)
         total = len(sortedstate)
         pos = 0
         for rev in sortedstate:
             pos += 1
             if state[rev] == -1:
                 ui.progress(_("rebasing"), pos, ("%d:%s" % (rev, repo[rev])),
                             _('changesets'), total)
                 storestatus(repo, originalwd, target, state, collapsef, keepf,
                                                     keepbranchesf, external)
                 p1, p2 = defineparents(repo, rev, target, state,
                                                         targetancestors)
                 if len(repo.parents()) == 2:
                     repo.ui.debug('resuming interrupted rebase\n')
                 else:
                     stats = rebasenode(repo, rev, p1, p2, state)
                     if stats and stats[3] > 0:
                         raise util.Abort(_('unresolved conflicts (see hg '
                                     'resolve, then hg rebase --continue)'))
                 updatedirstate(repo, rev, target, p2)
                 if not collapsef:
                     newrev = concludenode(repo, rev, p1, p2, extrafn=extrafn)
                 else:
                     # Skip commit if we are collapsing
                     repo.dirstate.setparents(repo[p1].node())
                     newrev = None
                 # Update the state
                 if newrev is not None:
                     state[rev] = repo[newrev].rev()
                 else:
                     if not collapsef:
                         ui.note(_('no changes, revision %d skipped\n') % rev)
                         ui.debug('next revision set to %s\n' % p1)
                         skipped.add(rev)
                     state[rev] = p1
 
         ui.progress(_('rebasing'), None)
         ui.note(_('rebase merging completed\n'))
 
         if collapsef and not keepopen:
             p1, p2 = defineparents(repo, min(state), target,
                                                         state, targetancestors)
             commitmsg = 'Collapsed revision'
             for rebased in state:
                 if rebased not in skipped and state[rebased] != nullmerge:
                     commitmsg += '\n* %s' % repo[rebased].description()
             commitmsg = ui.edit(commitmsg, repo.ui.username())
             newrev = concludenode(repo, rev, p1, external, commitmsg=commitmsg,
                                   extrafn=extrafn)
 
         if 'qtip' in repo.tags():
             updatemq(repo, state, skipped, **opts)
 
         if not keepf:
             # Remove no more useful revisions
             rebased = [rev for rev in state if state[rev] != nullmerge]
             if rebased:
                 if set(repo.changelog.descendants(min(rebased))) - set(state):
                     ui.warn(_("warning: new changesets detected "
                               "on source branch, not stripping\n"))
                 else:
                     # backup the old csets by default
                     repair.strip(ui, repo, repo[min(rebased)].node(), "all")
 
         clearstatus(repo)
         ui.note(_("rebase completed\n"))
         if os.path.exists(repo.sjoin('undo')):
             util.unlink(repo.sjoin('undo'))
         if skipped:
             ui.note(_("%d revisions have been skipped\n") % len(skipped))
     finally:
         release(lock, wlock)
 
 def rebasemerge(repo, rev, first=False):
     'return the correct ancestor'
     oldancestor = ancestor.ancestor
 
     def newancestor(a, b, pfunc):
         if b == rev:
             return repo[rev].parents()[0].rev()
         return oldancestor(a, b, pfunc)
 
     if not first:
         ancestor.ancestor = newancestor
     else:
         repo.ui.debug("first revision, do not change ancestor\n")
     try:
         stats = merge.update(repo, rev, True, True, False)
         return stats
     finally:
         ancestor.ancestor = oldancestor
 
 def checkexternal(repo, state, targetancestors):
     """Check whether one or more external revisions need to be taken in
     consideration. In the latter case, abort.
     """
     external = nullrev
     source = min(state)
     for rev in state:
         if rev == source:
             continue
         # Check externals and fail if there are more than one
         for p in repo[rev].parents():
             if (p.rev() not in state
                         and p.rev() not in targetancestors):
                 if external != nullrev:
                     raise util.Abort(_('unable to collapse, there is more '
                             'than one external parent'))
                 external = p.rev()
     return external
 
 def updatedirstate(repo, rev, p1, p2):
     """Keep track of renamed files in the revision that is going to be rebased
     """
     # Here we simulate the copies and renames in the source changeset
     cop, diver = copies.copies(repo, repo[rev], repo[p1], repo[p2], True)
     m1 = repo[rev].manifest()
     m2 = repo[p1].manifest()
     for k, v in cop.iteritems():
         if k in m1:
             if v in m1 or v in m2:
                 repo.dirstate.copy(v, k)
                 if v in m2 and v not in m1:
                     repo.dirstate.remove(v)
 
 def concludenode(repo, rev, p1, p2, commitmsg=None, extrafn=None):
     'Commit the changes and store useful information in extra'
     try:
         repo.dirstate.setparents(repo[p1].node(), repo[p2].node())
         ctx = repo[rev]
         if commitmsg is None:
             commitmsg = ctx.description()
         extra = {'rebase_source': ctx.hex()}
         if extrafn:
             extrafn(ctx, extra)
         # Commit might fail if unresolved files exist
         newrev = repo.commit(text=commitmsg, user=ctx.user(),
                              date=ctx.date(), extra=extra)
         repo.dirstate.setbranch(repo[newrev].branch())
         return newrev
     except util.Abort:
         # Invalidate the previous setparents
         repo.dirstate.invalidate()
         raise
 
 def rebasenode(repo, rev, p1, p2, state):
     'Rebase a single revision'
     # Merge phase
     # Update to target and merge it with local
     if repo['.'].rev() != repo[p1].rev():
         repo.ui.debug(" update to %d:%s\n" % (repo[p1].rev(), repo[p1]))
         merge.update(repo, p1, False, True, False)
     else:
         repo.ui.debug(" already in target\n")
     repo.dirstate.write()
     repo.ui.debug(" merge against %d:%s\n" % (repo[rev].rev(), repo[rev]))
     first = repo[rev].rev() == repo[min(state)].rev()
     stats = rebasemerge(repo, rev, first)
     return stats
 
 def defineparents(repo, rev, target, state, targetancestors):
     'Return the new parent relationship of the revision that will be rebased'
     parents = repo[rev].parents()
     p1 = p2 = nullrev
 
     P1n = parents[0].rev()
     if P1n in targetancestors:
         p1 = target
     elif P1n in state:
         if state[P1n] == nullmerge:
             p1 = target
         else:
             p1 = state[P1n]
     else: # P1n external
         p1 = target
         p2 = P1n
 
     if len(parents) == 2 and parents[1].rev() not in targetancestors:
         P2n = parents[1].rev()
         # interesting second parent
         if P2n in state:
             if p1 == target: # P1n in targetancestors or external
                 p1 = state[P2n]
             else:
                 p2 = state[P2n]
         else: # P2n external
             if p2 != nullrev: # P1n external too => rev is a merged revision
                 raise util.Abort(_('cannot use revision %d as base, result '
                         'would have 3 parents') % rev)
             p2 = P2n
     repo.ui.debug(" future parents are %d and %d\n" %
                             (repo[p1].rev(), repo[p2].rev()))
     return p1, p2
 
 def isagitpatch(repo, patchname):
     'Return true if the given patch is in git format'
     mqpatch = os.path.join(repo.mq.path, patchname)
     for line in patch.linereader(file(mqpatch, 'rb')):
         if line.startswith('diff --git'):
             return True
     return False
 
 def updatemq(repo, state, skipped, **opts):
     'Update rebased mq patches - finalize and then import them'
     mqrebase = {}
     mq = repo.mq
     for p in mq.applied:
         rev = repo[p.node].rev()
         if rev in state:
             repo.ui.debug('revision %d is an mq patch (%s), finalize it.\n' %
                                         (rev, p.name))
             mqrebase[rev] = (p.name, isagitpatch(repo, p.name))
 
     if mqrebase:
         mq.finish(repo, mqrebase.keys())
 
         # We must start import from the newest revision
         for rev in sorted(mqrebase, reverse=True):
             if rev not in skipped:
                 name, isgit = mqrebase[rev]
                 repo.ui.debug('import mq patch %d (%s)\n' % (state[rev], name))
                 mq.qimport(repo, (), patchname=name, git=isgit,
                                 rev=[str(state[rev])])
         mq.save_dirty()
 
 def storestatus(repo, originalwd, target, state, collapse, keep, keepbranches,
                                                                 external):
     'Store the current status to allow recovery'
     f = repo.opener("rebasestate", "w")
     f.write(repo[originalwd].hex() + '\n')
     f.write(repo[target].hex() + '\n')
     f.write(repo[external].hex() + '\n')
     f.write('%d\n' % int(collapse))
     f.write('%d\n' % int(keep))
     f.write('%d\n' % int(keepbranches))
     for d, v in state.iteritems():
         oldrev = repo[d].hex()
         newrev = repo[v].hex()
         f.write("%s:%s\n" % (oldrev, newrev))
     f.close()
     repo.ui.debug('rebase status stored\n')
 
 def clearstatus(repo):
     'Remove the status files'
     if os.path.exists(repo.join("rebasestate")):
         util.unlink(repo.join("rebasestate"))
 
 def restorestatus(repo):
     'Restore a previously stored status'
     try:
         target = None
         collapse = False
         external = nullrev
         state = {}
         f = repo.opener("rebasestate")
         for i, l in enumerate(f.read().splitlines()):
             if i == 0:
                 originalwd = repo[l].rev()
             elif i == 1:
                 target = repo[l].rev()
             elif i == 2:
                 external = repo[l].rev()
             elif i == 3:
                 collapse = bool(int(l))
             elif i == 4:
                 keep = bool(int(l))
             elif i == 5:
                 keepbranches = bool(int(l))
             else:
                 oldrev, newrev = l.split(':')
                 state[repo[oldrev].rev()] = repo[newrev].rev()
         skipped = set()
         # recompute the set of skipped revs
         if not collapse:
             seen = set([target])
             for old, new in sorted(state.items()):
                 if new != nullrev and new in seen:
                     skipped.add(old)
                 seen.add(new)
         repo.ui.debug('computed skipped revs: %s\n' % skipped)
         repo.ui.debug('rebase status resumed\n')
         return (originalwd, target, state, skipped,
                 collapse, keep, keepbranches, external)
     except IOError, err:
         if err.errno != errno.ENOENT:
             raise
         raise util.Abort(_('no rebase in progress'))
 
 def abort(repo, originalwd, target, state):
     'Restore the repository to its original state'
     if set(repo.changelog.descendants(target)) - set(state.values()):
         repo.ui.warn(_("warning: new changesets detected on target branch, "
                                                     "can't abort\n"))
         return -1
     else:
         # Strip from the first rebased revision
         merge.update(repo, repo[originalwd].rev(), False, True, False)
         rebased = filter(lambda x: x > -1 and x != target, state.values())
         if rebased:
             strippoint = min(rebased)
             # no backup of rebased cset versions needed
             repair.strip(repo.ui, repo, repo[strippoint].node())
         clearstatus(repo)
         repo.ui.warn(_('rebase aborted\n'))
         return 0
 
 def buildstate(repo, dest, src, base, detach):
     'Define which revisions are going to be rebased and where'
     targetancestors = set()
     detachset = set()
 
     if not dest:
         # Destination defaults to the latest revision in the current branch
         branch = repo[None].branch()
         dest = repo[branch].rev()
     else:
         dest = repo[dest].rev()
 
     # This check isn't strictly necessary, since mq detects commits over an
     # applied patch. But it prevents messing up the working directory when
     # a partially completed rebase is blocked by mq.
     if 'qtip' in repo.tags() and (repo[dest].node() in
                             [s.node for s in repo.mq.applied]):
         raise util.Abort(_('cannot rebase onto an applied mq patch'))
 
     if src:
         commonbase = repo[src].ancestor(repo[dest])
         if commonbase == repo[src]:
             raise util.Abort(_('source is ancestor of destination'))
         if commonbase == repo[dest]:
             raise util.Abort(_('source is descendant of destination'))
         source = repo[src].rev()
         if detach:
             # We need to keep track of source's ancestors up to the common base
             srcancestors = set(repo.changelog.ancestors(source))
             baseancestors = set(repo.changelog.ancestors(commonbase.rev()))
             detachset = srcancestors - baseancestors
-            detachset.remove(commonbase.rev())
+            detachset.discard(commonbase.rev())
     else:
         if base:
             cwd = repo[base].rev()
         else:
             cwd = repo['.'].rev()
 
         if cwd == dest:
             repo.ui.debug('source and destination are the same\n')
             return None
 
         targetancestors = set(repo.changelog.ancestors(dest))
         if cwd in targetancestors:
             repo.ui.debug('source is ancestor of destination\n')
             return None
 
         cwdancestors = set(repo.changelog.ancestors(cwd))
         if dest in cwdancestors:
             repo.ui.debug('source is descendant of destination\n')
             return None
 
         cwdancestors.add(cwd)
         rebasingbranch = cwdancestors - targetancestors
         source = min(rebasingbranch)
 
     repo.ui.debug('rebase onto %d starting from %d\n' % (dest, source))
     state = dict.fromkeys(repo.changelog.descendants(source), nullrev)
     state.update(dict.fromkeys(detachset, nullmerge))
     state[source] = nullrev
     return repo['.'].rev(), repo[dest].rev(), state
 
 def pullrebase(orig, ui, repo, *args, **opts):
     'Call rebase after pull if the latter has been invoked with --rebase'
     if opts.get('rebase'):
         if opts.get('update'):
             del opts['update']
             ui.debug('--update and --rebase are not compatible, ignoring '
                      'the update flag\n')
 
         cmdutil.bail_if_changed(repo)
         revsprepull = len(repo)
         origpostincoming = commands.postincoming
         def _dummy(*args, **kwargs):
             pass
         commands.postincoming = _dummy
         try:
             orig(ui, repo, *args, **opts)
         finally:
             commands.postincoming = origpostincoming
         revspostpull = len(repo)
         if revspostpull > revsprepull:
             rebase(ui, repo, **opts)
             branch = repo[None].branch()
             dest = repo[branch].rev()
             if dest != repo['.'].rev():
                 # there was nothing to rebase we force an update
                 hg.update(repo, dest)
     else:
         orig(ui, repo, *args, **opts)
 
 def uisetup(ui):
     'Replace pull with a decorator to provide --rebase option'
     entry = extensions.wrapcommand(commands.table, 'pull', pullrebase)
     entry[1].append(('', 'rebase', None,
                      _("rebase working directory to branch head"))
 )
 
 cmdtable = {
 "rebase":
         (rebase,
         [
         ('s', 'source', '',
          _('rebase from the specified changeset'), _('REV')),
         ('b', 'base', '',
          _('rebase from the base of the specified changeset '
            '(up to greatest common ancestor of base and dest)'),
          _('REV')),
         ('d', 'dest', '',
          _('rebase onto the specified changeset'), _('REV')),
         ('', 'collapse', False, _('collapse the rebased changesets')),
         ('', 'keep', False, _('keep original changesets')),
         ('', 'keepbranches', False, _('keep original branch names')),
         ('', 'detach', False, _('force detaching of source from its original '
                                 'branch')),
         ('c', 'continue', False, _('continue an interrupted rebase')),
         ('a', 'abort', False, _('abort an interrupted rebase'))] +
          templateopts,
         _('hg rebase [-s REV | -b REV] [-d REV] [options]\n'
           'hg rebase {-a|-c}'))
 }

tests/test-rebase-detach.t

@@ -1,193 +1,234 b''
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > graphlog=
   > rebase=
   > 
   > [alias]
   > tglog = log -G --template "{rev}: '{desc}' {branches}\n"
   > EOF
 
 
   $ hg init a
   $ cd a
 
   $ echo A > A
   $ hg ci -Am A
   adding A
 
   $ echo B > B
   $ hg ci -Am B
   adding B
 
   $ echo C > C
   $ hg ci -Am C
   adding C
 
   $ echo D > D
   $ hg ci -Am D
   adding D
 
   $ hg up -q -C 0
 
   $ echo E > E
   $ hg ci -Am E
   adding E
   created new head
 
   $ cd ..
 
 
 Rebasing D onto E detaching from C:
 
   $ hg clone -q -u . a a1
   $ cd a1
 
   $ hg tglog
   @  4: 'E'
   |
   | o  3: 'D'
   | |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg rebase --detach -s 3 -d 4
   saved backup bundle to $TESTTMP/a1/.hg/strip-backup/*-backup.hg (glob)
 
   $ hg tglog
   @  4: 'D'
   |
   o  3: 'E'
   |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg manifest
   A
   D
   E
 
   $ cd ..
 
 
 Rebasing C onto E detaching from B:
 
   $ hg clone -q -u . a a2
   $ cd a2
 
   $ hg tglog
   @  4: 'E'
   |
   | o  3: 'D'
   | |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg rebase --detach -s 2 -d 4
   saved backup bundle to $TESTTMP/a2/.hg/strip-backup/*-backup.hg (glob)
 
   $ hg tglog
   @  4: 'D'
   |
   o  3: 'C'
   |
   o  2: 'E'
   |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg manifest
   A
   C
   D
   E
 
   $ cd ..
 
 
 Rebasing B onto E using detach (same as not using it):
 
   $ hg clone -q -u . a a3
   $ cd a3
 
   $ hg tglog
   @  4: 'E'
   |
   | o  3: 'D'
   | |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg rebase --detach -s 1 -d 4
   saved backup bundle to $TESTTMP/a3/.hg/strip-backup/*-backup.hg (glob)
 
   $ hg tglog
   @  4: 'D'
   |
   o  3: 'C'
   |
   o  2: 'B'
   |
   o  1: 'E'
   |
   o  0: 'A'
   
   $ hg manifest
   A
   B
   C
   D
   E
 
   $ cd ..
 
 
 Rebasing C onto E detaching from B and collapsing:
 
   $ hg clone -q -u . a a4
   $ cd a4
 
   $ hg tglog
   @  4: 'E'
   |
   | o  3: 'D'
   | |
   | o  2: 'C'
   | |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg rebase --detach --collapse -s 2 -d 4
   saved backup bundle to $TESTTMP/a4/.hg/strip-backup/*-backup.hg (glob)
 
   $ hg tglog
   @  3: 'Collapsed revision
   |  * C
   |  * D'
   o  2: 'E'
   |
   | o  1: 'B'
   |/
   o  0: 'A'
   
   $ hg manifest
   A
   C
   D
   E
 
   $ cd ..
 
+Rebasing across null as ancestor
+  $ hg clone -q -U a a5
+
+  $ cd a5
+
+  $ echo x > x
+
+  $ hg add x
+
+  $ hg ci -m "extra branch"
+  created new head
+
+  $ hg tglog
+  @  5: 'extra branch'
+  
+  o  4: 'E'
+  |
+  | o  3: 'D'
+  | |
+  | o  2: 'C'
+  | |
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+  $ hg rebase --detach -s 1 -d tip
+  saved backup bundle to $TESTTMP/a5/.hg/strip-backup/*-backup.hg (glob)
+
+  $ hg tglog
+  @  5: 'D'
+  |
+  o  4: 'C'
+  |
+  o  3: 'B'
+  |
+  o  2: 'extra branch'
+  
+  o  1: 'E'
+  |
+  o  0: 'A'
+