add(decl, prefix, prefixes, result) {
    let prefixed = this.prefixed(decl.prop, prefix)
    if (
      this.isAlready(decl, prefixed) ||
      this.otherPrefixes(decl.value, prefix)
    ) {
      return undefined
    }
    return this.insert(decl, prefix, prefixes, result)
  }
Clone and add prefixes for declaration