mixin backdrop
    -
        let classes = {
            'o-backdrop':   true,
            'u-p-relative': true,
        }
        if (attributes.class) {
            classes[attributes.class] = true;
        }

    div(class=classes, style={ width: '100%', 'min-height': '5em', 'max-height': '40em' })
        block