Twig HTML Select Macro with optgroups

macros.twig

{% macro select (name, id, options, selected, required, includeBlank) %}
    <select name="{{ name }}" id="{{ id }}" {{ required ? 'required' : '' }}>
        {% if includeBlank %}
            <option value=""></option>
        {% endif %}
        {% for key, value in options %}
            {% if value is iterable %}
                <optgroup label="{{ key }}">
                    {% for subKey, subValue in value %}
                        <option value="{{ subKey }}" {{ subKey == selected ? 'selected' : '' }}>{{ subValue }}</option>
                    {% endfor %}
                </optgroup>
            {% else %}
                <option value="{{ key }}" {{ key == selected ? 'selected' : '' }}>{{ value }}</option>
            {% endif %}
        {% endfor %}
    </select>
{% endmacro %}