add `verbose-app`

beautiful-racket-lib/br/verbose-app.rkt

@@ -0,0 +1,10 @@
+#lang br
+(require (prefix-in br: (only-in br #%app)))
+(provide #%app)
+
+(define-macro (#%app APP ARG ...)
+  #'(let ()
+      (br:#%app displayln (br:#%app format "handling subexpressions in ~a" '(APP ARG ...)))
+      (define result (br:#%app APP ARG ...))
+      (br:#%app displayln (br:#%app format "evaluating ~a = ~a" '(APP ARG ...) result ))
+      result))