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diff --git a/one/seven.scm b/one/seven.scm
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+;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;; International. See
+;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (one seven)
+  #:export (tolerances-and-sqrt))
+
+(use-modules (one six))
+
+(define (gen-good-enough tolerance)
+  (lambda (guess x)
+    (< (abs (- (square guess) x)) tolerance)))
+
+(define (sqrt-iter-with-tolerance guess x tolerance)
+  (let ((good-enough? (gen-good-enough tolerance)))
+    (if (good-enough? guess x)
+        guess
+        (sqrt-iter-with-tolerance (improve guess x)
+                   x tolerance))))
+
+(define (sqrt-with-tolerance x tolerance)
+ (sqrt-iter-with-tolerance 1.0 x tolerance))
+
+(define (tolerances-and-sqrt x tolerance)
+  (let* ((guile-sqrt (sqrt x))
+         (custom-sqrt (sqrt-with-tolerance x tolerance)))
+    (if (eqv? guile-sqrt custom-sqrt)
+        (cons tolerance custom-sqrt)
+        (cons (cons tolerance custom-sqrt)
+              (tolerances-and-sqrt x (/ tolerance 10))))))