update book code

eBook/examples/chapter_6/defer_tracing.go

@@ -0,0 +1,22 @@
+package main
+
+import "fmt"
+
+func trace(s string)   { fmt.Println("entering:", s) }
+func untrace(s string) { fmt.Println("leaving:", s) }
+
+func a() {
+	trace("a")
+	defer untrace("a")
+	fmt.Println("in a")
+}
+
+func b() {
+	trace("b")
+	defer untrace("b")
+	fmt.Println("in b")
+	a()
+}
+func main() { 
+	b() 
+}