[racket] How ought I run the annotate function in gui-debugger/annotator on a datum?

I am trying to learn how to use the DrRacket debugger's annotate function.
My ultimate aim is to build a REPL that you can execute from within a
closure and have access to everything that's in scope. (see my StackOverflow
question <http://stackoverflow.com/q/19876940/585977> on that topic and
Greg Hendershott's well-researched answer) For the time being, I'm just
trying to explore how the annotate function works. You can see my first
exploratory attempt at using it, and the results,
here<http://pasterack.org/pastes/89>
.

The error, which is happing inside of the annotator, seems to arise when it
tries to pattern-match he application of string-append. The matcher is
looking for a #%plain-app and the expanded syntax I'm presenting to it
contains an #%app. I'm unsure if I should be expanding the syntax
differently so it comes out as a #%plain-app or if there's something else
I'm doing wrong to produce the syntax I'm feeding into the annotator. Does
anybody see where my error is?

I originally posted this question to SO, but Greg suggested I might have
more luck with it here.

Best,
Patrick

PS - if you're reading this, thanks the great PLAI class last year, SK!
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