diff --git a/lisp/random/problem.lisp b/lisp/random/problem.lisp
index e297504..ad4b69a 100644
--- a/lisp/random/problem.lisp
+++ b/lisp/random/problem.lisp
@@ -1,6 +1,11 @@
-(defun problem (n)
-  (if (< n 1) nil
-    (cons n (problem (- n 1)))))
+(load "../lang/functions.lisp")
 
-(setq y (problem 20))
-(setq x (problem 20000))
+(defun build (n lst)
+  (if (= n 0) lst
+    (build (- n 1) (cons (car lst) lst))))
+
+(defun build2 (n lst)
+  (if (= n 0) lst
+    (build2 (- n 1) (build 200 lst))))
+
+(length (build2 200 (build2 200 '(1 1 1 1 0 0 0 0))))
diff --git a/src/function/builtin/cons/LENGTH.java b/src/function/builtin/cons/LENGTH.java
index 1bcd156..3496bc9 100644
--- a/src/function/builtin/cons/LENGTH.java
+++ b/src/function/builtin/cons/LENGTH.java
@@ -1,12 +1,15 @@
 package function.builtin.cons;
 
 import static function.builtin.cons.LIST.makeList;
+import static recursion.tail.TailCalls.done;
+import static recursion.tail.TailCalls.tailCall;
 
 import java.math.BigInteger;
 
 import function.ArgumentValidator;
 import function.FunctionNames;
 import function.LispFunction;
+import recursion.tail.TailCall;
 import sexpression.Cons;
 import sexpression.LispNumber;
 
@@ -32,21 +35,21 @@ public class LENGTH extends LispFunction {
     public LispNumber call(Cons argumentList) {
         argumentValidator.validate(argumentList);
 
-        return callTailRecursive(BigInteger.ZERO, argumentList);
+        return callTailRecursive(BigInteger.ZERO, argumentList).invoke();
     }
 
     private LispNumber callWithoutArgumentValidation(Cons argumentList) {
-        return callTailRecursive(BigInteger.ZERO, argumentList);
+        return callTailRecursive(BigInteger.ZERO, argumentList).invoke();
     }
 
-    private LispNumber callTailRecursive(BigInteger accumulatedLength, Cons argumentList) {
+    private TailCall<LispNumber> callTailRecursive(BigInteger accumulatedLength, Cons argumentList) {
         Cons list = (Cons) argumentList.getFirst();
         Cons restOfList = makeList(list.getRest());
 
         if (list.isNull())
-            return new LispNumber(accumulatedLength);
+            return done(new LispNumber(accumulatedLength));
 
-        return callTailRecursive(increment(accumulatedLength), restOfList);
+        return tailCall(() -> callTailRecursive(increment(accumulatedLength), restOfList));
     }
 
     private BigInteger increment(BigInteger number) {
diff --git a/src/recursion/tail/TailCall.java b/src/recursion/tail/TailCall.java
new file mode 100644
index 0000000..0f8f7ef
--- /dev/null
+++ b/src/recursion/tail/TailCall.java
@@ -0,0 +1,22 @@
+package recursion.tail;
+
+import java.util.stream.Stream;
+
+@FunctionalInterface
+public interface TailCall<T> {
+
+    TailCall<T> apply();
+
+    default boolean isComplete() {
+        return false;
+    }
+
+    default T result() {
+        throw new UnsupportedOperationException();
+    }
+
+    default T invoke() {
+        return Stream.iterate(this, TailCall::apply).filter(TailCall::isComplete).findFirst().get().result();
+    }
+
+}
\ No newline at end of file
diff --git a/src/recursion/tail/TailCalls.java b/src/recursion/tail/TailCalls.java
new file mode 100644
index 0000000..482043c
--- /dev/null
+++ b/src/recursion/tail/TailCalls.java
@@ -0,0 +1,29 @@
+package recursion.tail;
+
+public class TailCalls {
+
+    public static <T> TailCall<T> tailCall(TailCall<T> nextCall) {
+        return nextCall;
+    }
+
+    public static <T> TailCall<T> done(T value) {
+        return new TailCall<T>() {
+
+            @Override
+            public boolean isComplete() {
+                return true;
+            }
+
+            @Override
+            public T result() {
+                return value;
+            }
+
+            @Override
+            public TailCall<T> apply() {
+                throw new UnsupportedOperationException();
+            }
+        };
+    }
+
+}
\ No newline at end of file