Improve fib tracer example (#17)

* Fix incorrectly hardcoded number in tracer fib example

* Change ordering in tracer example to provoke activation problem

* Care about results in the tracer example test

source/tap/tracer_example_test.go

@@ -75,20 +75,20 @@ func tracedFib(n int, scope *tap.TraceScope) (r int) {
 func fib(n int) int {
 	if scope := fibTracer.MaybeScope("wrapper"); scope != nil {
 		scope.Open(n)
-		r := tracedFib(5, scope)
+		r := tracedFib(n, scope)
 		scope.CloseCall(r)
 		return r
 	}
 	return untracedFib(n)
 }
 
 func ExampleTracer() {
-	// this one won't be traced since there is no watcher yet
-	fib(4)
-
 	// this just makes trace ids stable for the test
 	tap.ResetTraceID()
 
+	// this one won't be traced since there is no watcher yet
+	fmt.Printf("the 4th fib is %v\n", fib(4))
+
 	// this causes fibTracer's output to get printed to stdout; the use here is
 	// more complicated than you'd have in a real program to get stable test
 	// output.
@@ -101,17 +101,19 @@ func ExampleTracer() {
 	defer rep.Stop()
 
 	// this one will be traced since there's now a watcher
-	fib(5)
+	fmt.Printf("the 5th fib is %v\n", fib(5))
 
 	// this flushes and stops all watchers on the reported source; otherwise
 	// this function returns too quickly for even one of the emitted trace
 	// items to have been printed.
 	rep.Source().(source.DrainableSource).Drain()
 
 	// this one won't be traced since Drain deactivated the tracer
-	fib(6)
+	fmt.Printf("the 6th fib is %v\n", fib(6))
 
 	// Output:
+	// the 4th fib is 5
+	// the 5th fib is 8
 	// /tap/trace/fib: --> DATE TIME_0 [1::1] wrapper: 5
 	// /tap/trace/fib: --> DATE TIME_1 [1:1:2] fib(5)
 	// /tap/trace/fib: --> DATE TIME_2 [1:2:3] fib(4)
@@ -144,6 +146,7 @@ func ExampleTracer() {
 	// /tap/trace/fib: <-- DATE TIME_29 [1:2:12] return 3
 	// /tap/trace/fib: <-- DATE TIME_30 [1:1:2] return 8
 	// /tap/trace/fib: <-- DATE TIME_31 [1::1] return 8
+	// the 6th fib is 13
 }
 
 // timeElider is here just to normalize output for the test