add(decl, prefix) {
    if (!decl._autoprefixerValues) {
      decl._autoprefixerValues = {}
    }
    let value = decl._autoprefixerValues[prefix] || this.value(decl)

    let before
    do {
      before = value
      value = this.replace(value, prefix)
      if (value === false) return
    } while (value !== before)

    decl._autoprefixerValues[prefix] = value
  }
Save values with next prefixed token