add(rule, prefix) {
    let prefixed = prefix + rule.name

    let already = rule.parent.some(
      i => i.name === prefixed && i.params === rule.params
    )
    if (already) {
      return undefined
    }

    let cloned = this.clone(rule, { name: prefixed })
    return rule.parent.insertBefore(rule, cloned)
  }
Clone and add prefixes for at-rule