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diff --git a/number-guesser.scm b/number-guesser.scm
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+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Example game:                      ;;
+;; scheme@(guile-user)> (start 60 70) ;;
+;; $21 = 65                           ;;
+;; scheme@(guile-user)> (bigger)      ;;
+;; $22 = 68                           ;;
+;; scheme@(guile-user)> (bigger)      ;;
+;; $23 = 69                           ;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; also see (info "(guile) Definition")
+
+(define lower 1)
+(define upper 100)
+
+(define (start n m)
+  (set! lower (min n m))
+  (set! upper (max n m))
+  (guess))
+
+;;;; helpers
+(define (guess)
+  (quotient (+ lower upper) 2))
+
+(define (smaller)
+  (set! upper (max lower (- (guess) 1)))
+  (guess))
+
+(define (bigger)
+  (set! lower (min upper (+ (guess) 1)))
+  (guess))