improve the xml serializer

while here:
- do some refactoring in BaseExpression

Makefile

@@ -17,11 +17,20 @@ samples: FORCE
 	$(MAKE) -C samples
 
 
+samples-gcc11: FORCE
+	env CXX=g++11 CXXFLAGS="-Wl,-rpath=/usr/local/lib/gcc11/ -Wall -pedantic -O0 -g -std=c++20 -fmax-errors=1 -I../src -I../../pikotools/src -I/usr/local/include" $(MAKE) -C src
+	env CXX=g++11 CXXFLAGS="-Wl,-rpath=/usr/local/lib/gcc11/ -Wall -pedantic -O0 -g -std=c++20 -fmax-errors=1 -I../src -I../../pikotools/src -I/usr/local/include" $(MAKE) -C samples
+
+
 clean: FORCE
 	$(MAKE) -C src clean
 	$(MAKE) -C samples clean
 
 
+cleanall: clean
+	$(MAKE) -C ../pikotools clean
+
+
 depend: FORCE
 	$(MAKE) -C src depend
 	$(MAKE) -C samples depend