--- a/sources/mystring.h	Fri Feb 05 11:36:38 2021 +0200
+++ b/sources/mystring.h	Mon Apr 10 10:42:45 2023 +0300
@@ -32,6 +32,7 @@
 #include <deque>
 #include <string>
 #include <stdarg.h>
+#include <optional>
 #include "basics.h"
 #include "list.h"