title: Hackable scheme implementation with decent C++ FFI
body: <p>I have tried: scheme48, chicken, gambit, clojure.</p>
  
  <p>I am looking for a scheme implementation that's readable (the implementation itself), and has a decent C++ ffi, and good debugging suport (so when I get an exception, I should get popped into a recursive repl).</p>
  
  <p>What suggestions do people have?</p>