/** Returns true if a Vary header contains an asterisk. Such responses cannot be cached. */
    fun Response.hasVaryAll(): Boolean = "*" in headers.varyFields()

    /**
     * Returns the names of the request headers that need to be checked for equality when caching.
     */
    private fun Headers.varyFields(): Set<String> {
      var result: MutableSet<String>? = null
      for (i in 0 until size) {