diff --git a/README.md b/README.md
index c507d0c..3036594 100644
--- a/README.md
+++ b/README.md
@@ -1,13 +1,22 @@
 # Brainfuck2Scheme
 
 A simple compiler from Brainfuck to R5RS. The compiler will output a Scheme
-list that is a lambda with two arguments:
+list that is a lambda with three arguments:
 
 * `data`: The data vector.
 * `dptr`: The initial data pointer.
+* `debugger`: Debugger function
 
 To turn the list into an executable Scheme function, just give it to
-`eval`.
+`eval`. `(execute scheme len)` will run the Scheme code in `scheme`
+with a data vector of length `len`.
+
+This dialect of Brainfuck supports `#` to call the `debugger` procedure
+with `data` and `dptr` as arguments.
+
+Since Brainfuck programs become Scheme procedures, you can modularize
+Brainfuck code and (ab)use the debugger for things like procedure calls
+and foreign libraries.
 
 ## How It Works
 
@@ -24,6 +33,7 @@ Data access brainfuck instructions are translated like
 * `<` -> `(set! dptr (- dptr 1))`
 * `.` -> `(display (vector-ref data dptr))`
 * `,` -> `(vector-set! data dptr (read-char))`
+* `#` -> `(debugger data dptr)`
 
 Branches are trickier. Basically, all code that will be executed in a block
 is in a lambda. Given `[code...]`, the `code...` will be compiled to a
diff --git a/bf2s.scm b/bf2s.scm
index 60def3e..b865c70 100644
--- a/bf2s.scm
+++ b/bf2s.scm
@@ -44,6 +44,8 @@
         ((#\,) (compile (cdr lst)
                         (cons '(vector-set! data dptr
                                             (char->integer (read-char))) ins)))
+        ((#\#) (compile (cdr lst)
+                        (cons '(debugger data dptr) ins)))
         ((#\[) (let ((rest (compile (cdr lst) '())))
                  (if (not (pair? rest))
                      (error "unmatched [")
@@ -62,7 +64,7 @@
         (else (compile (cdr lst) ins)))))
 
 (define (brainfuck->scheme str)
-  `(lambda (data dptr)
+  `(lambda (data dptr debugger)
      (,(compile (string->list str) '()))))
 
 (define (brainfuck->scheme-from-file filename)
@@ -74,5 +76,8 @@
              str
              (loop (string-append str (read-line port)))))))))
 
-(define (execute scheme)
-  ((eval scheme) (make-vector 4096) 0))
+(define (execute scheme len)
+  ((eval scheme) (make-vector len) 0
+                   (lambda (data dptr)
+                     (display (list data dptr))
+                     (newline))))