<div class="panel-group" id="blog-archive">
  {{ range $i, $e := .CurrentSection.Pages.GroupByPublishDate "2006" }}
  <div class="panel panel-default">
    <div class="panel-heading">
      <h4 class="panel-title">
      <a data-toggle="collapse" data-parent="#archive" href="#collapse-{{ .Key }}">
      {{ .Key }}</a>
      </h4>
    </div>
    <div id="collapse-{{ .Key }}" class="panel-collapse collapse{{ if eq $i 0 }} in{{ end }}">
      <div class="panel-body">
        <ul class="list-group">
          {