code: func TTL(ttl uint8) NetworkChecker {
  	return func(t *testing.T, h []header.Network) {
  		var v uint8
  		switch ip := h[0].(type) {
  		case header.IPv4:
  			v = ip.TTL()
  		case header.IPv6:
  			v = ip.HopLimit()
  		}
  		if v != ttl {
  			t.Fatalf("Bad TTL, got %v, want %v", v, ttl)
  		}
  	}
  }
docs: // TTL creates a checker that checks the TTL (ipv4) or HopLimit (ipv6).