While fixing problems with the sidebar tabs (see #1328), I found out that the template code is duplicated in no fewer than five Twig template files, with slight variations:

• Skosmos/view/changes.twig

  Lines 7 to 32 in 290296c

  	<ul class="nav nav-tabs{% if vocab.config.showChangeList and vocab.config.groupClassURI %} nav-four-wide{% endif %}">
  	{% set css_class = ['nav-link'] %}
  	{% if request.vocab.config.showAlphabeticalIndex %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts alphabetically" %}</h3>
  	<li id="alpha" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/index{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% if vocab.config.showChangeList and vocab.config.groupClassURI %}{% trans 'A-Z' %}{% else %}{% trans "Alpha-nav" %}{% endif %}</a></li>
  	{% endif %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts hierarchically" %}</h3>
  	{% if active_tab == 'hierarchy' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="hierarchy{% if not vocab.config.showHierarchy %}-disabled{% endif %}" class="nav-item"><a class="{{ css_class|join(' ') }}" href="#" id="hier-trigger"{% if not vocab.config.showHierarchy %} title="{% trans 'hierarchy-disabled-help' %}"{% endif %}>{% trans "Hier-nav" %}</a></li>
  	{% if vocab.config.groupClassURI %}
  	{% set css_class = ['nav-link'] %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts and groupings hierarchically" %}</h3>
  	<li id="groups" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/groups{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Group-nav" %}</a></li>
  	{% endif %}
  	{% if vocab.config.showChangeList %}
  	{% if vocab.config.showDeprecatedChanges %}
  	{% set css_class = ['nav-link'] %}
  	{% set css_class = css_class|merge(['active']) %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions including removed" %}</h3>
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-and-deprecations-nav" %}</a></li>
  	{% else %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions" %}</h3>
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-nav" %}</a></li>
  	{% endif %}
  	{% endif %}
  	</ul>

• Skosmos/view/group-contents.twig

  Lines 7 to 33 in 290296c

  	<div class="sidebar-buttons">
  	<h2 class="sr-only">{% trans "Sidebar listing: list and traverse vocabulary contents by a criterion" %}</h2>
  	<ul class="nav nav-tabs{% if vocab.config.showChangeList and vocab.config.groupClassURI and vocab.config.showAlphabeticalIndex %} nav-four-wide{% endif %}">
  	{% set css_class = ['nav-link'] %}
  	{% if request.vocab.config.showAlphabeticalIndex %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts alphabetically" %}</h3>
  	<li id="alpha" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/index{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% if vocab.config.showChangeList and vocab.config.groupClassURI %}{% trans 'A-Z' %}{% else %}{% trans "Alpha-nav" %}{% endif %}</a></li>
  	{% endif %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts hierarchically" %}</h3>
  	<li id="hierarchy{% if not vocab.config.showHierarchy %}-disabled{% endif %}" class="nav-item"><a class="{{ css_class|join(' ') }}" href="#" id="hier-trigger"{% if not vocab.config.showHierarchy %} title="{% trans 'hierarchy-disabled-help' %}"{% endif %}>{% trans "Hier-nav" %}</a></li>
  	{% if vocab.config.groupClassURI %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts and groupings hierarchically" %}</h3>
  	{% if search_results|first.isGroup %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="groups" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/groups{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Group-nav" %}</a></li>
  	{% endif %}
  	{% if vocab.config.showChangeList %}
  	{% if vocab.config.showDeprecatedChanges %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions including removed" %}</h3>
  	<li id="changes"><a href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-and-deprecations-nav" %}</a></li>
  	{% else %}
  	{% set css_class = ['nav-link'] %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions" %}</h3>
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-nav" %}</a></li>
  	{% endif %}
  	{% endif %}
  	</ul>
  	</div>

• Skosmos/view/group-index.twig

  Lines 6 to 32 in 290296c

  	<div class="sidebar-buttons">
  	<h2 class="sr-only">{% trans "Sidebar listing: list and traverse vocabulary contents by a criterion" %}</h2>
  	<ul class="nav nav-tabs{% if vocab.config.showChangeList and vocab.config.groupClassURI and vocab.config.showAlphabeticalIndex %} nav-four-wide{% endif %}">
  	{% set css_class = ['nav-link'] %}
  	{% if request.vocab.config.showAlphabeticalIndex %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts alphabetically" %}</h3>
  	<li id="alpha" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/index{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% if vocab.config.showChangeList and vocab.config.groupClassURI %}{% trans 'A-Z' %}{% else %}{% trans "Alpha-nav" %}{% endif %}</a></li>
  	{% endif %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts hierarchically" %}</h3>
  	<li id="hierarchy{% if not vocab.config.showHierarchy %}-disabled{% endif %}" class="nav-item"><a class="{{ css_class|join(' ') }}" href="#" id="hier-trigger"{% if not vocab.config.showHierarchy %} title="{% trans 'hierarchy-disabled-help' %}"{% endif %}>{% trans "Hier-nav" %}</a></li>
  	{% if vocab.config.groupClassURI %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts and groupings hierarchically" %}</h3>
  	{% set css_class = css_class|merge(['active']) %}
  	<li id="groups" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/groups{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Group-nav" %}</a></li>
  	{% endif %}
  	{% if vocab.config.showChangeList %}
  	{% if vocab.config.showDeprecatedChanges %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions including removed" %}</h3>
  	<li id="changes"><a href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-and-deprecations-nav" %}</a></li>
  	{% else %}
  	{% set css_class = ['nav-link'] %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions" %}</h3>
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-nav" %}</a></li>
  	{% endif %}
  	{% endif %}
  	</ul>
  	</div>

• Skosmos/view/light.twig

  Lines 44 to 86 in 290296c

  	<div class="sidebar-buttons">
  	{% set sidebarHeading = request.page != 'search' ? "Sidebar listing: list and traverse vocabulary contents by a criterion" : "Sidebar listing: vocabulary search options" %}
  	<h2 class="sr-only">{% trans sidebarHeading %}</h2>
  	<ul class="nav nav-tabs{% if vocab.config.showChangeList and vocab.config.groupClassURI and vocab.config.showAlphabeticalIndex %} nav-four-wide{% endif %}">
  	{% if request.page != 'search' %}
  	{% set css_class = ['nav-link'] %}
  	{% if request.vocab.config.showAlphabeticalIndex %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts alphabetically" %}</h3>
  	{% if search_results|length != 1 or (term and search_results|length == 1) %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="alpha" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/index{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% if vocab.config.showChangeList and vocab.config.groupClassURI %}{% trans'A-Z'%}{% else %}{% trans "Alpha-nav" %}{% endif %}</a></li>
  	{% endif %}
  	{% set css_class = ['nav-link'] %}
  	{% set disabledHierarchy = not vocab.config.showHierarchy and not search_results|length == 1 %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts hierarchically" %}</h3>
  	{% if search_results|length == 1 and not term %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="hierarchy{%- if disabledHierarchy %}-disabled{% endif -%}" class="nav-item">
  	<a class="{{ css_class|join(' ') }}" href="#" id="hier-trigger"
  	{% if disabledHierarchy%} title="{% trans 'hierarchy-disabled-help' %}"{% endif %}
  	>{% trans "Hier-nav" %}
  	</a>
  	</li>
  	{% set css_class = ['nav-link'] %}
  	{% if vocab.config.groupClassURI %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts and groupings hierarchically" %}</h3>
  	<li id="groups" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/groups{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Group-nav" %}</a></li>
  	{% endif %}
  	{% if vocab.config.showChangeList %}
  	{% if vocab.config.showDeprecatedChanges %}
  	{% if active_tab == 'new' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions including removed" %}</h3>
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-and-deprecations-nav" %}</a></li>
  	{% else %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions" %}</h3>
  	{% set css_class = ['nav-link'] %}
  	{% if active_tab == 'new' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-nav" %}</a></li>
  	{% endif %}
  	{% endif %}
  	{% else %}
  	<h3 class="sr-only">{% trans "Search options tab" %}</h3>
  	<li id="limit" class="nav-item"><p>{% trans "Search options" %}</p></li>
  	{% endif %}
  	</ul>

• Skosmos/view/vocab.twig

  Lines 6 to 37 in 290296c

  	<div class="sidebar-buttons">
  	<h2 class="sr-only">{% trans "Sidebar listing: list and traverse vocabulary contents by a criterion" %}</h2>
  	<ul class="nav nav-tabs{% if vocab.config.showChangeList and vocab.config.groupClassURI and vocab.config.showAlphabeticalIndex %} nav-four-wide{% endif %}">
  	{% set css_class = ['nav-link'] %}
  	{% if request.vocab.config.showAlphabeticalIndex %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts alphabetically" %}</h3>
  	{% if active_tab == 'alphabetical' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="alpha" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/index{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% if vocab.config.showChangeList and vocab.config.groupClassURI %}{% trans 'A-Z' %}{% else %}{% trans "Alpha-nav" %}{% endif %}</a></li>
  	{% endif %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts hierarchically" %}</h3>
  	{% set css_class = ['nav-link'] %}
  	{% if active_tab == 'hierarchy' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="hierarchy{% if not vocab.config.showHierarchy %}-disabled{% endif %}" class="nav-item"><a class="{{ css_class|join(' ') }}" href="#" id="hier-trigger"{% if not vocab.config.showHierarchy %} title="{% trans 'hierarchy-disabled-help' %}"{% endif %}>{% trans "Hier-nav" %}</a></li>
  	{% if vocab.config.groupClassURI %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts and groupings hierarchically" %}</h3>
  	<li id="groups" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/groups{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Group-nav" %}</a></li>
  	{% endif %}
  	{% if vocab.config.showChangeList %}
  	{% if vocab.config.showDeprecatedChanges %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions including removed" %}</h3>
  	{% set css_class = ['nav-link'] %}
  	{% if active_tab == 'new' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-and-deprecations-nav" %}</a></li>
  	{% else %}
  	<h3 class="sr-only">{% trans "List vocabulary concepts by newest additions" %}</h3>
  	{% set css_class = ['nav-link'] %}
  	{% if active_tab == 'new' %}{% set css_class = css_class|merge(['active']) %}{% endif %}
  	<li id="changes" class="nav-item"><a class="{{ css_class|join(' ') }}" href="{{ request.vocabid }}/{{ request.lang }}/new{% if request.contentLang != request.lang %}?clang={{ request.contentLang }}{% endif %}">{% trans "Changes-nav" %}</a></li>
  	{% endif %}
  	{% endif %}
  	</ul>
  	</div>

Code duplication is bad for many reasons. These should really be consolidated into just one place (for example, a separate template file called tabs.twig or sidebar-tabs.twig) but the variations need to be ironed out making sure that functionality stays the same or improves.