WIP: background section

chapters/03_background/listings/classificationmarkers.cpp

@@ -0,0 +1,6 @@
+// Continuation of previous example
+if (sum == 100) {
+  fail_marker_positive();
+} else {
+  fail_marker_negative();
+}

chapters/03_background/listings/tracefence.cpp

@@ -0,0 +1,8 @@
+fail_start_trace();
+
+int sum = 0;
+for (int i = 0; i < 100; ++i) {
+  ++sum;
+}
+
+fail_stop_trace();