code: func Listener(l net.Listener) net.Listener {
  	return keepaliveListener{l.(*net.TCPListener)}
  }
docs: // Listener returns a net.Listener that enables TCP keep-alive timeouts on
  // accepted connections. It allows detection of dead TCP connections (e.g.
  // closing laptop mid-download) to eventually go away. Derived from the Go
  // net/http package.