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 descendant of target ref name

            if not c.is_range and 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

            elif c.pull_request.is_closed():

                c.update_msg = _('This pull request has been closed and can not be updated.')

            else: # look for descendants of PR head on source branch in org repo

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

                                                         revs[0], c.cs_branch_name)

                if len(avail_revs) > 1: # more than just revs[0]

                    # also show changesets that not are descendants but would be merged in

                    targethead = other_scm_instance.get_changeset(c.a_branch_name).raw_id

                    if org_scm_instance.path != other_scm_instance.path:

                        # Note: org_scm_instance.path must come first so all

                        # valid revision numbers are 100% org_scm compatible

                        # - both for avail_revs and for revset results

                        hgrepo = unionrepo.unionrepository(org_scm_instance.baseui,

                                                           org_scm_instance.path,

                                                           other_scm_instance.path)

                    else:

                        hgrepo = org_scm_instance._repo

                                           avail_revs, revs[0], targethead))

                    c.update_msg = _('This pull request can be updated with changes on %s:') % c.cs_branch_name

                else:

                    show = set()

                    avail_revs = set() # drop revs[0]

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

                # TODO: handle branch heads that not are tip-most

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

                if brevs:

                    # also show changesets that are on branch but neither ancestors nor descendants

                    show.update(org_scm_instance._repo.revs('::%ld - ::%ld - ::%s', brevs, avail_revs, c.a_branch_name))

                    show.add(revs[0]) # make sure graph shows this so we can see how they relate

                    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))

                avail_show = sorted(show, reverse=True)

        elif org_scm_instance.alias == 'git':

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

        c.avail_revs = avail_revs

        c.avail_cs = [org_scm_instance.get_changeset(r) for r in avail_show]

        c.avail_jsdata = json.dumps(graph_data(org_scm_instance, avail_show))

          </div>

          <div class="input pr-do-edit ac" style="display:none">

               ${h.text('owner', class_="large", value=c.pull_request.owner.username, placeholder=_('Username'))}

               <div id="owner_completion_container"></div>

          </div>

        </div>

        <div class="field">

            <div class="label-summary">

              <label>${_('Update')}:</label>

            </div>

            <div class="input">

              <div class="msg-div">${c.update_msg}</div>

              %if c.avail_revs:

              <div id="updaterevs" style="max-height:200px; overflow-y:auto; overflow-x:hidden; margin-bottom: 10px; padding: 1px 0">

                <div style="height:0">

                  <canvas id="avail_graph_canvas" style="width:0"></canvas>

                </div>

                <table id="updaterevs-table" class="noborder" style="padding-left:50px">

                  %for cnt, cs in enumerate(c.avail_cs):

                    <tr id="chg_available_${cnt+1}">

                      %if cs.revision == c.cs_ranges[-1].revision:

                        <td>

                          %if editable:

                          %endif

                        </td>

                        <td colspan="2">${_("Current revision - no change")}</td>

                      %else:

                        <td>

                          %if editable and cs.revision in c.avail_revs:

                            ${h.radio(name='updaterev', value=cs.raw_id)}

                          %endif

                        </td>

                        <td>${h.link_to(h.show_id(cs),h.url('changeset_home',repo_name=c.cs_repo.repo_name,revision=cs.raw_id))}</td>

                        <td><div class="message" style="white-space:normal; height:1.1em; max-width: 500px; padding:0">${h.urlify_commit(cs.message, c.repo_name)}</div></td>

                      %endif

                    </tr>

                  %endfor

                </table>

              </div>

              %endif

              <div class="msg-div">${c.update_msg_other}</div>

            </div>

        </div>

      </div>

    </div>

    ## REVIEWERS

    <div style="float:left; border-left:1px dashed #eee">