diff --git a/lisp/finance/interest-compounder-test.lisp b/lisp/finance/interest-compounder-test.lisp
index aad6e27..7c73145 100644
--- a/lisp/finance/interest-compounder-test.lisp
+++ b/lisp/finance/interest-compounder-test.lisp
@@ -7,74 +7,97 @@
 
       (defun principal-initialized ()
         (setf compounder (interest-compounder 1000 0))
-        (assert= 1000 (funcall compounder :get-principal)))
+        (assert= 1000 (call compounder :get-principal)))
 
       (defun interest-rate-initialized ()
         (setf compounder (interest-compounder 0 10))
-        (assert= 10 (funcall compounder :get-interest-rate)))
+        (assert= 10 (call compounder :get-interest-rate)))
 
       (defun many-years-with-no-interest-rate ()
         (setf compounder (interest-compounder 1000 0))
-        (funcall compounder :move-forward-years 83)
-        (assert= 1000 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 83)
+        (assert= 1000 (call compounder :get-principal)))
 
       (defun no-years-with-positive-interest-rate ()
         (setf compounder (interest-compounder 1000 10))
-        (assert= 1000 (funcall compounder :get-principal)))
+        (assert= 1000 (call compounder :get-principal)))
 
       (defun one-year-with-positive-interest-rate ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-one-year)
-        (assert= 105000 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 1)
+        (assert= 105000 (call compounder :get-principal)))
 
       (defun two-years-with-positive-interest-rate ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-one-year)
-        (funcall compounder :move-forward-one-year)
-        (assert= 110250 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 2)
+        (assert= 110250 (call compounder :get-principal)))
 
       (defun three-years-with-positive-interest-rate ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-years 3)
-        (assert= 115763 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 3)
+        (assert= 115763 (call compounder :get-principal)))
 
       (defun four-years-with-positive-interest-rate ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-years 4)
-        (assert= 121551 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 4)
+        (assert= 121551 (call compounder :get-principal)))
 
       (defun one-year-with-negative-interest-rate ()
         (setf compounder (interest-compounder 100000 -5))
-        (funcall compounder :move-forward-years 1)
-        (assert= 95000 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 1)
+        (assert= 95000 (call compounder :get-principal)))
 
       (defun two-years-with-negative-interest-rate ()
         (setf compounder (interest-compounder 100000 -5))
-        (funcall compounder :move-forward-years 2)
-        (assert= 90250 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 2)
+        (assert= 90250 (call compounder :get-principal)))
 
       (defun three-years-with-negative-interest-rate ()
         (setf compounder (interest-compounder 100000 -5))
-        (funcall compounder :move-forward-years 3)
-        (assert= 85737 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 3)
+        (assert= 85737 (call compounder :get-principal)))
 
       (defun four-years-with-negative-interest-rate ()
         (setf compounder (interest-compounder 100000 -5))
-        (funcall compounder :move-forward-years 4)
-        (assert= 81450 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years 4)
+        (assert= 81450 (call compounder :get-principal)))
 
       (defun negative-number-of-years-does-nothing ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-years -4)
-        (assert= 100000 (funcall compounder :get-principal))
-        (funcall compounder :move-forward-years 1)
-        (funcall compounder :move-forward-years -4)
-        (assert= 105000 (funcall compounder :get-principal)))
+        (call compounder :move-forward-years -4)
+        (assert= 100000 (call compounder :get-principal))
+        (call compounder :move-forward-years 1)
+        (call compounder :move-forward-years -4)
+        (assert= 105000 (call compounder :get-principal)))
 
       (defun zero-number-of-years-does-nothing ()
         (setf compounder (interest-compounder 100000 5))
-        (funcall compounder :move-forward-years 0)
-        (assert= 100000 (funcall compounder :get-principal))
-        (funcall compounder :move-forward-years 1)
-        (funcall compounder :move-forward-years 0)
-        (assert= 105000 (funcall compounder :get-principal))))))
+        (call compounder :move-forward-years 0)
+        (assert= 100000 (call compounder :get-principal))
+        (call compounder :move-forward-years 1)
+        (call compounder :move-forward-years 0)
+        (assert= 105000 (call compounder :get-principal)))
+
+      (defun variable-interest-rate ()
+        (setf compounder (interest-compounder 100000 5))
+        (call compounder :move-forward-years 2)
+        (call compounder :set-interest-rate 10)
+        (call compounder :move-forward-years 2)
+        (assert= 133403 (call compounder :get-principal)))
+
+      (defun years-are-updated ()
+        (setf compounder (interest-compounder 100000 5))
+        (call compounder :move-forward-years 27)
+        (assert= 27 (call compounder :get-years-passed)))
+
+      (defun negative-number-of-years-does-not-update-years ()
+        (setf compounder (interest-compounder 100000 5))
+        (call compounder :move-forward-years 27)
+        (call compounder :move-forward-years -2)
+        (assert= 27 (call compounder :get-years-passed)))
+
+      (defun zero-number-of-years-does-not-update-years ()
+        (setf compounder (interest-compounder 100000 5))
+        (call compounder :move-forward-years 27)
+        (call compounder :move-forward-years 0)
+        (assert= 27 (call compounder :get-years-passed))))))
diff --git a/lisp/finance/interest-compounder.lisp b/lisp/finance/interest-compounder.lisp
index 932fb52..d04f82a 100644
--- a/lisp/finance/interest-compounder.lisp
+++ b/lisp/finance/interest-compounder.lisp
@@ -3,12 +3,17 @@
 (defun interest-compounder (initial-principal initial-interest-rate)
   (let ((private) (public)
         (principal initial-principal)
-        (interest-rate initial-interest-rate))
+        (interest-rate initial-interest-rate)
+        (years-passed 0))
 
     (setf private
       (eval
         (dlambda
 
+          (:add-years (years)
+            (if (> years 0)
+              (setf years-passed (+ years-passed years))))
+
           (:percent-of-number (n percentage)
             (if (> percentage 0)
               (/ (+ (* n percentage) 50) 100)
@@ -26,14 +31,19 @@
       (eval
         (dlambda
 
+          (:get-years-passed ()
+            years-passed)
+
           (:get-principal ()
             principal)
 
           (:get-interest-rate ()
             interest-rate)
 
-          (:move-forward-one-year ()
-            (call private :compound-interest 1))
+          (:set-interest-rate (new-interest-rate)
+            (setf interest-rate new-interest-rate))
 
           (:move-forward-years (years)
-            (call private :compound-interest years)))))))
+            (progn
+              (call private :add-years years)
+              (call private :compound-interest years))))))))