$def with (q, placeholder="")

$ placeholder = placeholder or _('Search')

<div class="searchbox">
    <input type="text" class="searchbox__input" name="q" size="100" placeholder="$placeholder" aria-label="$_('Search')" value="$q">
    <div>
        <input type="submit" value="" class="searchbox__btn-icon" aria-label="$_('Search')">
    </div>
</div>