{# Will hold all the base CSS classes to add to the root element #}
{% set base_css_classes = ['o-button-group'] %}

{# Grow modifier CSS class #}
{% if grow %}
  {% set base_css_classes = base_css_classes|merge(['o-button-group--grow']) %}
{% endif %}

{# Concatenate the base CSS classes #}
{% set base_css_classes = base_css_classes|join(' ') %}

{# The Button Group template #}
<div class="{{ base_css_classes }}{% if class %} {{ class }}{% endif %}">
  {% block content %}{% endblock %}
</div>