added tutorials for adding a traversal tutorial and a tutorial about project structure and ast.xml. Also added tutorials from other people I found in the source code and a concepts directory with phm and ref counting also added a faq file

tutorials/gcov.md

+# Introduction
+
+`gcov` is a tool that counts how often each lines is executed. This is useful
+to uncover complexity issues.
+
+# Compilation
+
+First compile with coverage enabled.
+
+````
+mkdir build_c
+cd build_c
+cmake -DGCOV=ON .. && make -j10
+```
+
+Now remove all reports: after all we are not interested in the results
+from compiling the preludes. Do this after all programs you tested.
+
+```
+find -name \*gcda -exec rm {} \;
+```
+
+# Common issues and solutions
+
+1. Building a linked list with append instead of prepend. This has quadratic
+   behaviour. Consider prepending if the order does not matter (e.g. with
+   variable declarations), or storing the end of your linked list.
+2. Searching in linked lists, e.g.
+```
+    node *n = ...;
+    while (!condition(n)) {
+        n = ..._NEXT(n);
+    }
+    return n;
+```
+   Consider creating a lookup table.
+
+# Example
+
+To be safe, we again delete all gcda files
+
+```
+find -name \*gcda -exec rm {} \;
+```
+
+## Houston we have a problem!
+
+```
+gcov $(find -name tree_compound.c.o)
+vi tree_compound.c.gcov
+```
+
+        -:  703:node *
+     6309:  704:TCappendArgs (node *arg_chain, node *arg)
+        -:  705:{
+        -:  706:    node *ret;
+        -:  707:
+     6309:  708:    DBUG_ENTER ();
+        -:  709:
+    6309*:  710:    DBUG_ASSERT (((arg_chain == NULL) || (NODE_TYPE (arg_chain) == N_arg)),
+        -:  711:                 "First argument of TCappendArgs() has wrong node type.");
+    6309*:  712:    DBUG_ASSERT (((arg == NULL) || (NODE_TYPE (arg) == N_arg)),
+        -:  713:                 "Second argument of TCappendArgs() has wrong node type.");
+        -:  714:
+    53555:  715:    APPEND (ret, node *, ARG, arg_chain, arg);
+        -:  716:
+     6309:  717:    DBUG_RETURN (ret);
+        -:  718:}
+
+Call `gcov` on all files using `TCappendArgs`.
+
+```
+grep -l TCappendArgs -r ../src | xargs -n1 basename  | sed 's/$/\.o/g' | xargs -n1 find -name | xargs -n1 gcov
+```
+
+Filter out the files where `TCappendArgs` was never executed:
+
+```
+grep TCappendArgs *gcov | grep -v \# | grep -v "        -"
+```
+
+This outputs
+
+```
+eliminate_duplicate_fundef_args.c.gcov:     5563:  172:            newargs = TCappendArgs (newargs, arg_node);
+emr_loop_optimisation.c.gcov:        2:  421:                          = TCappendArgs (INFO_ARGS (arg_info),
+emr_loop_optimisation.c.gcov:        1:  531:              = TCappendArgs (FUNDEF_ARGS (arg_node), INFO_ARGS (arg_info));
+loop_scalarization.c.gcov:      168:  400:    DBUG_RETURN (TCappendArgs (new_args, old_args));
+tree_compound.c.gcov:     6309:  704:TCappendArgs (node *arg_chain, node *arg)
+wlpropagation.c.gcov:      465:  239:          = TCappendArgs (INFO_NEWLACFUNARGS (arg_info), copy);
+wlpropagation.c.gcov:      110:  493:          = TCappendArgs (INFO_NEWLACFUNARGS (arg_info),
+```
+
+We fix the quadratic behaviour (see merge request 325) and end up with
+
+```
+        -:  703:node *
+      171:  704:TCappendArgs (node *arg_chain, node *arg)
+        -:  705:{
+        -:  706:    node *ret;
+        -:  707:
+      171:  708:    DBUG_ENTER ();
+        -:  709:
+     171*:  710:    DBUG_ASSERT (((arg_chain == NULL) || (NODE_TYPE (arg_chain) == N_arg)),
+        -:  711:                 "First argument of TCappendArgs() has wrong node type.");
+     171*:  712:    DBUG_ASSERT (((arg == NULL) || (NODE_TYPE (arg) == N_arg)),
+        -:  713:                 "Second argument of TCappendArgs() has wrong node type.");
+        -:  714:
+      313:  715:    APPEND (ret, node *, ARG, arg_chain, arg);
+        -:  716:
+      171:  717:    DBUG_RETURN (ret);
+```