old_revisions = set(old_pull_request.revisions)

        revisions = [cs.raw_id for cs in cs_ranges]

        new_revisions = [r for r in revisions if r not in old_revisions]

        lost = old_revisions.difference(revisions)

        infos = ['This is an update of %s "%s".' %

                 (h.canonical_url('pullrequest_show', repo_name=old_pull_request.other_repo.repo_name,

                      pull_request_id=old_pull_request.pull_request_id),

                  old_pull_request.title)]

        if lost:

            for r in old_pull_request.revisions:

                if r in lost:

                    rev_desc = org_repo.get_changeset(r).message.split('\n')[0]

                    infos.append('  %s "%s"' % (h.short_id(r), rev_desc))

        if new_revisions:

            for r in reversed(revisions):

                if r in new_revisions:

                    rev_desc = org_repo.get_changeset(r).message.split('\n')[0]

                    infos.append('  %s %s' % (h.short_id(r), h.shorter(rev_desc, 80)))

            if ancestor_rev == other_rev:

                infos.append(_("Ancestor didn't change - show diff since previous version:"))

                infos.append(h.canonical_url('compare_url',

                                 repo_name=org_repo.repo_name, # other_repo is always same as repo_name

                                 org_ref_type='rev', org_ref_name=h.short_id(org_rev), # use old org_rev as base

                                 other_ref_type='rev', other_ref_name=h.short_id(new_org_rev),

                                 )) # note: linear diff, merge or not doesn't matter

        return redirect(pull_request.url())

    # pullrequest_post for PR editing

    @LoginRequired()

    @NotAnonymous()

    @HasRepoPermissionAnyDecorator('repository.read', 'repository.write',

                                   'repository.admin')

    def post(self, repo_name, pull_request_id):

        pull_request = PullRequest.get_or_404(pull_request_id)

        if pull_request.is_closed():

            raise HTTPForbidden()

        #only owner or admin can update it

        owner = pull_request.author.user_id == c.authuser.user_id

        repo_admin = h.HasRepoPermissionAny('repository.admin')(c.repo_name)

        if not (h.HasPermissionAny('hg.admin') or repo_admin or owner):

            raise HTTPForbidden()

        _form = PullRequestPostForm()().to_python(request.POST)

        reviewers_ids = [int(s) for s in _form['review_members']]

        if _form['updaterev']:

            return self.create_update(pull_request,

                                      _form['updaterev'],

                c.cs_branch_name = org_scm_instance.get_changeset(c.cs_ref_name).branch # use ref_type ?

            c.a_branch_name = c.a_ref_name

            if c.a_ref_type != 'branch':

                try:

                    c.a_branch_name = other_scm_instance.get_changeset(c.a_ref_name).branch # use ref_type ?

                except EmptyRepositoryError:

                    c.a_branch_name = 'null' # not a branch name ... but close enough

            # candidates: descendants of old head that are on the right branch

            #             and not are the old head itself ...

            #             and nothing at all if old head is a descendent of target ref name

            if other_scm_instance._repo.revs('present(%s)::&%s', c.cs_ranges[-1].raw_id, c.a_branch_name):

                c.update_msg = _('This pull request has already been merged to %s.') % c.a_branch_name

                arevs = org_scm_instance._repo.revs('%s:: & branch(%s) - %s',

                                                    revs[0], c.cs_branch_name, revs[0])

                if arevs:

                    arevs = sorted(arevs, reverse=True)

                else:

                    c.update_msg = _('No changesets found for updating this pull request.')

            brevs = org_scm_instance._repo.revs('%s - %d - %ld', c.cs_branch_name, revs[0], arevs)

            if brevs:

                c.update_msg_other = _('Note: Branch %s has another head: %s.') % (c.cs_branch_name,

                    h.short_id(org_scm_instance.get_changeset((max(brevs))).raw_id))

        elif org_scm_instance.alias == 'git':

            c.update_msg = _("Git pull requests don't support updates yet.")

        c.available = [org_scm_instance.get_changeset(r) for r in arevs]

        raw_ids = [x.raw_id for x in c.cs_ranges]

        c.cs_comments = c.cs_repo.get_comments(raw_ids)