}
        for (varyField in value.split(',')) {
          result.add(varyField.trim())
        }
      }
      return result ?: emptySet()
    }

    /**
     * Returns the subset of the headers in this's request that impact the content of this's body.
     */
    fun Response.varyHeaders(): Headers {
      // Use the request headers sent over the network, since that's what the response varies on.