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+====== How to use C++20 modules in GNU g++-11 ======
+using gcc11-20210124-32.exe (Windows build from [[http://www.equation.com/servlet/equation.cmd?fa=fortran|equation.com]]) (Warning: This is experimental!)
+===== module definition =====
+''%%#include%%'' headers in the global module fragment
+<code cpp>
+module;
+#include <iostream>
+// ...
+</code>
+before ''module'' //''module_name''//'';'' or ''import'' them after that:
+<code cpp greetings.cpp>
+export module greetings;
+import <iostream>;
+export auto say_hello()
+{
+    std::cout << "Hello, modules!\n";
+}
+</code>
+===== module interface unit =====
+Every module consists of exactly one module interface unit ''export module'' //''module_name''//'';'' after which it declares which names should be ''export''ed, and eventually other module implementation units (source files) starting with ''module'' //''module_name''//'';'' (without ''export'').
+===== import module =====
+by its module name if you want to use the contents of a module
+<code cpp main.cpp>
+import greetings;
+int main()
+{
+    say_hello();
+}
+</code>
+===== Prepare (std) header unit(s) =====
+before compiling modularized source code containing e.g. ''import <iostream>;''
+<code>
+g++ -fmodules-ts -std=c++20 -c -x c++-system-header iostream
+</code>
+Other options are ''c++-header'' and ''c++-user-header'' (uses ''#include'' path).
+===== Compile program =====
+with option ''-fmodules-ts'' in ''c++20'' mode:
+<code>
+g++ greetings.cpp main.cpp -fmodules-ts -std=c++20
+</code>
+Done! Start program:
+<code>
+> a.exe
+Hello, modules!
+</code>
+===== Further reading =====
+  * Nathan Sidwell: C++ Modules. A Brief Tour. Overload 28(159) 19-23, October 2020. https://accu.org/journals/overload/28/159/sidwell