diff --git a/fitnesse/FitNesseRoot/RecentChanges.wiki b/fitnesse/FitNesseRoot/RecentChanges.wiki
index 2e1d183..8447b41 100644
--- a/fitnesse/FitNesseRoot/RecentChanges.wiki
+++ b/fitnesse/FitNesseRoot/RecentChanges.wiki
@@ -1,3 +1,4 @@
+|TranscendentalLisp.FinanceUnitTests||11:03:11 Wed, Mar 08, 2017|
 |TranscendentalLisp.LexicalClosures||16:39:59 Tue, Mar 07, 2017|
 |FrontPage||10:19:45 Tue, Mar 07, 2017|
 |TranscendentalLisp||10:14:02 Tue, Mar 07, 2017|
diff --git a/fitnesse/FitNesseRoot/TranscendentalLisp/FinanceUnitTests.wiki b/fitnesse/FitNesseRoot/TranscendentalLisp/FinanceUnitTests.wiki
new file mode 100644
index 0000000..cac682f
--- /dev/null
+++ b/fitnesse/FitNesseRoot/TranscendentalLisp/FinanceUnitTests.wiki
@@ -0,0 +1,14 @@
+---
+Test
+---
+Unit tests for the interest-compounder class.
+
+| script | lisp interpreter fixture                                   |
+| clear functions and symbols                                         |
+| check  | evaluate file | lisp/finance/interest-compounder-test.lisp | =~/T$/ |
+
+Unit tests for the compound-interest function.
+
+| script | lisp interpreter fixture                                 |
+| clear functions and symbols                                       |
+| check  | evaluate file | lisp/finance/compound-interest-test.lisp | =~/T$/ |
diff --git a/fitnesse/FitNesseRoot/TranscendentalLisp/ExternalFunctionUnitTests.wiki b/fitnesse/FitNesseRoot/TranscendentalLisp/LangUnitTests.wiki
similarity index 100%
rename from fitnesse/FitNesseRoot/TranscendentalLisp/ExternalFunctionUnitTests.wiki
rename to fitnesse/FitNesseRoot/TranscendentalLisp/LangUnitTests.wiki
diff --git a/lisp/finance/interest-compounder-test.lisp b/lisp/finance/interest-compounder-test.lisp
index 4cb61c0..122bdeb 100644
--- a/lisp/finance/interest-compounder-test.lisp
+++ b/lisp/finance/interest-compounder-test.lisp
@@ -100,4 +100,24 @@
         (setq compounder (interest-compounder 100000 5))
         (call compounder :move-forward-years 27)
         (call compounder :move-forward-years 0)
-        (assert= 27 (call compounder :get-years-passed))))))
+        (assert= 27 (call compounder :get-years-passed)))
+
+      (defun make-contribution ()
+        (setq compounder (interest-compounder 100000 5))
+        (call compounder :make-contribution 2000)
+        (assert= 102000 (call compounder :get-principal)))
+
+      (defun make-several-contributions ()
+        (setq compounder (interest-compounder 100000 5))
+        (call compounder :make-contribution 2000)
+        (call compounder :make-contribution 2000)
+        (call compounder :make-contribution 1000)
+        (assert= 105000 (call compounder :get-principal)))
+
+      (defun make-several-contributions-and-earn-interest ()
+        (setq compounder (interest-compounder 100000 5))
+        (call compounder :make-contribution 2000)
+        (call compounder :move-forward-years 2)
+        (call compounder :make-contribution 2000)
+        (call compounder :move-forward-years 2)
+        (assert= 126187 (call compounder :get-principal))))))
diff --git a/lisp/finance/interest-compounder.lisp b/lisp/finance/interest-compounder.lisp
index d279085..65a601e 100644
--- a/lisp/finance/interest-compounder.lisp
+++ b/lisp/finance/interest-compounder.lisp
@@ -1,49 +1,58 @@
 (load "../lang/dlambda.lisp")
 
-(defun interest-compounder (initial-principal initial-interest-rate)
-  (let ((private) (public)
-        (principal initial-principal)
-        (interest-rate initial-interest-rate)
-        (years-passed 0))
+(let ((static))
 
-    (setq private
-      (eval
-        (dlambda
+  (setq static
+    (eval
+      (dlambda
 
-          (:add-years (years)
-            (if (> years 0)
-              (setq years-passed (+ years-passed years))))
+        (:percent-of-number (n percentage)
+          (if (> percentage 0)
+            (/ (+ (* n percentage) 50) 100)
+            (/ (- (* n percentage) 50) 100))))))
 
-          (:percent-of-number (n percentage)
-            (if (> percentage 0)
-              (/ (+ (* n percentage) 50) 100)
-              (/ (- (* n percentage) 50) 100)))
+  (defun interest-compounder (initial-principal initial-interest-rate)
+    (let ((private) (public)
+          (principal initial-principal)
+          (interest-rate initial-interest-rate)
+          (years-passed 0))
 
-          (:compound-interest (years)
-            (if (> years 0)
+      (setq private
+        (eval
+          (dlambda
+
+            (:add-years (years)
+              (if (> years 0)
+                (setq years-passed (+ years-passed years))))
+
+           (:compound-interest (years)
+              (if (> years 0)
+                (begin
+                  (setq principal
+                    (+ principal
+                      (call static :percent-of-number principal interest-rate)))
+                  (call private :compound-interest (- years 1))))))))
+
+      (setq public
+        (eval
+          (dlambda
+
+            (:get-years-passed ()
+              years-passed)
+
+            (:get-principal ()
+              principal)
+
+            (:get-interest-rate ()
+              interest-rate)
+
+            (:set-interest-rate (new-interest-rate)
+              (setq interest-rate new-interest-rate))
+
+            (:make-contribution (contribution)
+              (setq principal (+ principal contribution)))
+
+            (:move-forward-years (years)
               (begin
-                (setq principal
-                  (+ principal
-                    (call private :percent-of-number principal interest-rate)))
-                (call private :compound-interest (- years 1))))))))
-
-    (setq public
-      (eval
-        (dlambda
-
-          (:get-years-passed ()
-            years-passed)
-
-          (:get-principal ()
-            principal)
-
-          (:get-interest-rate ()
-            interest-rate)
-
-          (:set-interest-rate (new-interest-rate)
-            (setq interest-rate new-interest-rate))
-
-          (:move-forward-years (years)
-            (begin
-              (call private :add-years years)
-              (call private :compound-interest years))))))))
+                (call private :compound-interest years)
+                (call private :add-years years)))))))))
diff --git a/lisp/lang/dlambda.lisp b/lisp/lang/dlambda.lisp
index ce293cc..ab8f792 100644
--- a/lisp/lang/dlambda.lisp
+++ b/lisp/lang/dlambda.lisp
@@ -1,5 +1,7 @@
 (load "functions.lisp")
 
+;; A special function based on the dlambda macro presented in "Let Over Lambda" by Doug Hoyte.
+
 (let
   ((add-method-clause
     (lambda (method)
diff --git a/lisp/object/multiple-methods.lisp b/lisp/object/multiple-methods.lisp
index 23d2aa6..dfd7950 100644
--- a/lisp/object/multiple-methods.lisp
+++ b/lisp/object/multiple-methods.lisp
@@ -1,3 +1,5 @@
+;; This is based on an example presented in "Let Over Lambda" by Doug Hoyte.
+
 (defun counter-class ()
   (let ((counter 0))
     (lambda (msg)
diff --git a/lisp/object/static.lisp b/lisp/object/static.lisp
index 831f0cb..414cb87 100644
--- a/lisp/object/static.lisp
+++ b/lisp/object/static.lisp
@@ -1,3 +1,5 @@
+;; This is based on an example presented in "Let Over Lambda" by Doug Hoyte.
+
 (let ((direction 'up))
   (defun toggle-counter-direction ()
     (setq direction
diff --git a/lisp/unit/unit-test.lisp b/lisp/unit/unit-test.lisp
index ef55170..ae09b09 100644
--- a/lisp/unit/unit-test.lisp
+++ b/lisp/unit/unit-test.lisp
@@ -1,10 +1,9 @@
 (defun run-unit-test (unit-test)
   (if (call unit-test)
-    (progn
+    (begin
       (print (cons t unit-test))
       t)
-
-    (progn
+    (begin
       (print (cons 'f unit-test))
       nil)))
 
@@ -15,12 +14,12 @@
       (run-test-suite (cdr test-suite)))))
 
 (defun unit (test-suite)
-  (eval (cons 'and (run-test-suite test-suite))))
+  (apply 'and (run-test-suite test-suite)))
 
 (defun assert (comparison expected actual)
   (if (call comparison expected actual)
     t
-    (progn
+    (begin
       (print (list expected 'is 'not comparison actual))
       nil)))