diff --git a/bin/tests/evaluation_of_default_args.slime b/bin/tests/evaluation_of_default_args.slime
new file mode 100644
index 0000000..aae5501
--- /dev/null
+++ b/bin/tests/evaluation_of_default_args.slime
@@ -0,0 +1,7 @@
+((lambda (:keys k1 :defaults-to (+ 1 2 3))
+   (assert (= k1 6))))
+
+((lambda (:keys k1 :defaults-to ())
+   (when k1
+     (assert ()))
+   (assert (= k1 ()))))
diff --git a/manual/manual.org b/manual/manual.org
index 98771c0..d0216d4 100644
--- a/manual/manual.org
+++ b/manual/manual.org
@@ -198,6 +198,7 @@ The programmer can also define their own special forms using =special-lambda= an
 be explained later.
 
 * Symbols and keywords
+* Truthyness
 * Lambdas
 Slime  allows for  creating anonymous  functions called  *lambdas*. We  did not  talk about  binding
 variables,  we will  do  this in  [[Define]], but  we  can still  use  lambdas now.  Remember that  Lisp
@@ -327,18 +328,19 @@ call to that function will look like.
 
 ** Functions with keyword arguments
 A sometimes more convenient way of passing arguments to a function is using keyword arguments. Using
-keyword arguments a function call could look like this: \[\texttt{(function :arg1 value1 :arg2
-value2)}\] here the function accepts two arguments named =arg1= and =arg2=. The user of this
+keyword  arguments a  function call  could look  like this:  \[\texttt{(function :arg1  value1 :arg2
+value2)}\]  here the  function accepts  two arguments  named  =arg1= and  =arg2=. The  user of  this
 function can see more clearly excatly which argument will be assigned wich value. This notation also
-allows for switching the argument order. The following function call is equivalent to the call
+allows for  switching the  argument order.  The following function  call is  equivalent to  the call
 above. \[\texttt{(function :arg2 value2 :arg1 value1)}\].
 
 For this to work however, the function must be defined to accept these keyword arguments. To do this
 the special marker  =:keys= has to be  inserted into the argument  list of a =lambda=  or a function
-=define=. All following arguments *must* be supplied as keyword arguments, /unless/ they are also
-supplied with a default value, in which case they do not need to be supplied. To attach a default
-value to a keyword argument, insert =:defaults-to <value>= after the keyword argument name. An
-example of all of this can be seen in [[code:keyword-args]].
+=define=. All following  arguments *must* be supplied  as keyword arguments, /unless/  they are also
+supplied with a  default value, in which case they  do not need to be supplied.  To attach a default
+value to  a keyword  argument, insert  =:defaults-to <value>=  after the  keyword argument  name. An
+example of all of  this can be seen in [[code:keyword-args]]. Important note:  keyword arguments must be
+defined and supplied after all the regular arguments.
 
 {{{slime_header}}}
 #+name: code:keyword-args
@@ -361,10 +363,41 @@ example of all of this can be seen in [[code:keyword-args]].
 : 54
 : 54
 
+** Functions with rest arguments
+If the programmer wants to  create a function that can accept any number  of arguments, they can use
+the =rest= argument. It is defined after the  special marker =:rest= and after the rest argument, no
+other arguments can be  defined. In the execution of the fuction, the  rest arguent will be assigned
+to a list containing all the supplied values. The  rest argument can be used in conjunction with the
+other argument types, regular arguments and keyword arguments.
 
+{{{slime_header}}}
+#+name: code:rest-args
+#+caption: A more complex functoin definition using keyword arguments
+#+begin_src slime
+  (define (execute-operation operation
+                             :keys
+                             do-logging :defaults-to ()
+                             :rest values)
+    (define result (apply operation values))
+    (when do-logging
+      (printf "Executing operation"
+              operation
+              "agains values yielded:"
+              result))
+    result)
+
+  (printf (execute-operation '+ 1 2 3))
+  (printf (execute-operation '*
+                             :do-logging t
+                             10 11))
+#+end_src
 
+#+RESULTS: code:rest-args
+: evaluates to =>
+: 6
+: Executing operation [C-function] agains values yielded: 110
+: 110
 
-** Functions with rest arguments
 
 * Environments
 * Built-in functions
diff --git a/src/error.cpp b/src/error.cpp
index 2fef0ba..df7155c 100644
--- a/src/error.cpp
+++ b/src/error.cpp
@@ -14,7 +14,7 @@ proc create_error(const char* c_file_name, int c_file_line, Lisp_Object* type, S
     delete_error();
     debug_break();
 
-    visualize_lisp_machine();
+    // visualize_lisp_machine();
 
     using Globals::error;
     error = new(Error);
diff --git a/src/eval.cpp b/src/eval.cpp
index 6cb3252..a7cbb83 100644
--- a/src/eval.cpp
+++ b/src/eval.cpp
@@ -24,11 +24,11 @@ proc apply_arguments_to_function(Lisp_Object* arguments, Function* function) ->
 
     if (arguments == Memory::nil)
         goto checks;
+
     // keyword arguments: use all given ones and keep track of the
     // added ones (array list), if end of parameters in encountered or
     // something that is not a keyword is encountered or a keyword
     // that is not recognized is encoutered, jump out of the loop.
-
     while (Memory::get_type(arguments->value.pair.first) == Lisp_Object_Type::Keyword) {
         // check if this one is even an accepted keyword
         bool accepted = false;
@@ -80,8 +80,9 @@ proc apply_arguments_to_function(Lisp_Object* arguments, Function* function) ->
         }
 
         // if not set it and then add it to the array list
-        try sym = Memory::get_or_create_lisp_object_symbol(arguments->value.pair.first->value.symbol.identifier),
-        define_symbol(sym, arguments->value.pair.rest->value.pair.first, new_env);
+        try sym = Memory::get_or_create_lisp_object_symbol(arguments->value.pair.first->value.symbol.identifier);
+        // NOTE(Felix): It seems we do not need to evaluate the argument here...
+        try define_symbol(sym, arguments->value.pair.rest->value.pair.first, new_env);
 
         append_to_array_list(read_in_keywords, arguments->value.pair.first->value.symbol.identifier);
 
@@ -257,9 +258,11 @@ proc parse_argument_list(Lisp_Object* arguments, Function* function) -> void {
                 // would be an error
                 next = next->value.pair.rest;
                 if (Memory::get_type(next) == Lisp_Object_Type::Pair) {
+                    Lisp_Object* ret;
+                    try_void ret = eval_expr(next->value.pair.first, function->parent_environment);
                     append_to_keyword_argument_list(function->keyword_arguments,
                                                     arguments->value.pair.first->value.symbol.identifier,
-                                                    next->value.pair.first);
+                                                    ret);
                     arguments = next->value.pair.rest;
                 } else {
                     create_parsing_error("Expecting a value after 'defaults-to'");
@@ -426,6 +429,7 @@ proc eval_expr(Lisp_Object* node, Environment* env) -> Lisp_Object* {
 proc is_truthy(Lisp_Object* expression, Environment* env) -> bool {
     Lisp_Object* result;
     try result = eval_expr(expression, env);
+
     return result != Memory::nil;
 }
 
diff --git a/src/memory.cpp b/src/memory.cpp
index cd4dbfd..4610f5d 100644
--- a/src/memory.cpp
+++ b/src/memory.cpp
@@ -283,6 +283,9 @@ namespace Memory {
     }
 
     proc copy_lisp_object(Lisp_Object* n) -> Lisp_Object* {
+        if (n == nil || n == t)
+            return n;
+
         Lisp_Object* target;
         try target = create_lisp_object();
         *target = *n;
diff --git a/src/testing.cpp b/src/testing.cpp
index eaf1550..a2e4085 100644
--- a/src/testing.cpp
+++ b/src/testing.cpp
@@ -635,6 +635,7 @@ proc run_all_tests() -> bool {
 
     printf("\n-- Test Files --\n");
 
+    invoke_test_script("evaluation_of_default_args");
     invoke_test_script("lexical_scope");
     invoke_test_script("class_macro");
     invoke_test_script("sicp");