diff --git a/bin/pre.slime b/bin/pre.slime
index c815b44..e04e0f2 100644
--- a/bin/pre.slime
+++ b/bin/pre.slime
@@ -127,11 +127,13 @@ by the key 'from' and ends with the number defined in 'to'."
 (defun range-while (:keys from :defaults-to 0 to)
   "Returns a sequence of numbers starting with the number defined
 by the key 'from' and ends with the number defined in 'to'."
-  (define result (list from))
+  (define result (list (copy from)))
+  (define head result)
   (mutate from (incr from))
   (while (< from to)
     (prog
-     (append result (copy from))
+     (mutate head (pair (first head) (pair (copy from) nil)))
+     (define head (rest head))
      (mutate from (incr from))))
   result)
 
diff --git a/bin/test.slime b/bin/test.slime
index 053a891..c8c55d5 100644
--- a/bin/test.slime
+++ b/bin/test.slime
@@ -1,4 +1,4 @@
-(when 1 (breakpoint))
+;; (when 1 (breakpoint))
 
 ;; (if (eval 1)
 ;;     (apply prog ((breakpoint)))
@@ -7,3 +7,9 @@
 ;; (if (eval 1)
 ;;     (eval (pair prog ((breakpoint)))))
 ;;   nil))
+
+
+(defun ! (n)
+  (if (< n 2)
+      1
+    (* n (! (- 1 n)))))
diff --git a/build/slime.exe.dbg b/build/slime.exe.dbg
index d5b5597..ace130a 100644
Binary files a/build/slime.exe.dbg and b/build/slime.exe.dbg differ
diff --git a/src/ast.c b/src/ast.c
index 90085c1..389c3ee 100644
--- a/src/ast.c
+++ b/src/ast.c
@@ -126,6 +126,7 @@ typedef enum {
     Built_In_Addition,
     Built_In_And,
     Built_In_Breakpoint,
+    Built_In_Copy,
     Built_In_Define,
     Built_In_Division,
     Built_In_Equal,
@@ -169,6 +170,7 @@ char* Built_In_Name_to_string(Built_In_Name name) {
     case Built_In_Addition:       return "+";
     case Built_In_And:            return "and";
     case Built_In_Breakpoint:     return "breakpoint";
+    case Built_In_Copy:           return "copy";
     case Built_In_Define:         return "define";
     case Built_In_Division:       return "/";
     case Built_In_Equal:          return "=";
@@ -294,6 +296,7 @@ Ast_Node* create_ast_node_built_in_function(char* name) {
     else if (string_equal(name, ">="))           type = Built_In_Greater_Equal;
     else if (string_equal(name, "and"))          type = Built_In_And;
     else if (string_equal(name, "breakpoint"))   type = Built_In_Breakpoint;
+    else if (string_equal(name, "copy"))         type = Built_In_Copy;
     else if (string_equal(name, "define"))       type = Built_In_Define;
     else if (string_equal(name, "error"))        type = Built_In_Error;
     else if (string_equal(name, "eval"))         type = Built_In_Eval;
@@ -337,3 +340,9 @@ Ast_Node* create_ast_node_pair(Ast_Node* first, Ast_Node* rest) {
     node->value.pair->rest = rest;
     return node;
 }
+
+Ast_Node* copy_ast_node(Ast_Node* n) {
+    Ast_Node* target = new(Ast_Node);
+    *target = *n;
+    return target;
+}
diff --git a/src/env.c b/src/env.c
index 791228b..b1f7c59 100644
--- a/src/env.c
+++ b/src/env.c
@@ -6,10 +6,10 @@ typedef enum {
 
 struct Environment {
     struct Environment* parent;
-    Environment_Type	type;
+    Environment_Type    type;
 
-    int	      capacity;
-    int	      next_index;
+    int       capacity;
+    int       next_index;
     // TODO(Felix): Use a hashmap here.
     char**     keys;
     Ast_Node** values;
@@ -22,12 +22,12 @@ Environment* create_child_environment(Environment* parent, Environment_Type type
 
     int start_capacity = 16;
 
-    env->type	    = type;
-    env->parent	    = parent;
+    env->type       = type;
+    env->parent     = parent;
     env->capacity   = start_capacity;
     env->next_index = 0;
-    env->keys	    =	  (char**)malloc(start_capacity * sizeof(char*));
-    env->values	    = (Ast_Node**)malloc(start_capacity * sizeof(Ast_Node*));
+    env->keys       =     (char**)malloc(start_capacity * sizeof(char*));
+    env->values     = (Ast_Node**)malloc(start_capacity * sizeof(Ast_Node*));
 
     return env;
 }
@@ -38,12 +38,12 @@ Environment* create_empty_environment(Environment_Type type) {
 
 void define_symbol(Ast_Node* symbol, Ast_Node* value, Environment* env) {
     if (env->type == Environment_Type_Macro) {
-	// NOTE(Felix): we know we have a parent because every
-	// environment has a parent except the top level environment.
-	// However the top level environment is not a let-environment,
-	// so we would not land here
-	define_symbol(symbol, value, env->parent);
-	return;
+        // NOTE(Felix): we know we have a parent because every
+        // environment has a parent except the top level environment.
+        // However the top level environment is not a let-environment,
+        // so we would not land here
+        define_symbol(symbol, value, env->parent);
+        return;
     }
 
     // NOTE(Felix): right now we are simply adding the symol at the
@@ -52,9 +52,9 @@ void define_symbol(Ast_Node* symbol, Ast_Node* value, Environment* env) {
     // latest defined one first, but a bit messy. Later we should use
     // a hashmap here. @refactor
     if (env->next_index == env->capacity) {
-	env->capacity *= 2;
-	env->keys   =	  (char**)realloc(env->keys,   env->capacity * sizeof(char*));
-	env->values = (Ast_Node**)realloc(env->values, env->capacity * sizeof(Ast_Node*));
+        env->capacity *= 2;
+        env->keys   =     (char**)realloc(env->keys,   env->capacity * sizeof(char*));
+        env->values = (Ast_Node**)realloc(env->values, env->capacity * sizeof(Ast_Node*));
     }
 
     env->keys  [env->next_index] = symbol->value.symbol->identifier;
@@ -64,8 +64,8 @@ void define_symbol(Ast_Node* symbol, Ast_Node* value, Environment* env) {
 
 void define_macro_symbol(Ast_Node* symbol, Ast_Node* value, Environment* env) {
     if (env->type != Environment_Type_Macro) {
-	create_error(Error_Type_Unknown_Error, symbol);
-	return;
+        create_error(Error_Type_Unknown_Error, symbol);
+        return;
     }
 
     env->type = Environment_Type_Lambda;
@@ -77,19 +77,19 @@ void print_environment(Environment* env);
 
 Ast_Node* lookup_symbol_in_this_envt(Symbol* sym, Environment* env) {
     for (int i = env->next_index - 1; i >= 0; --i)
-	if (string_equal(env->keys[i], sym->identifier))
-	    return env->values[i];
+        if (string_equal(env->keys[i], sym->identifier))
+            return env->values[i];
     return nullptr;
 }
 
 Ast_Node* lookup_symbol_from_lambda_env(Symbol* sym, Environment* env) {
     Ast_Node* result;
     do {
-	if (env->type != Environment_Type_Lambda) {
-	    result = lookup_symbol_in_this_envt(sym, env);
-	    if (result) return result;
-	}
-	env = env->parent;
+        if (env->type != Environment_Type_Lambda) {
+            result = lookup_symbol_in_this_envt(sym, env);
+            if (result) return result;
+        }
+        env = env->parent;
     } while (env);
     return nullptr;
 }
@@ -97,22 +97,22 @@ Ast_Node* lookup_symbol_from_lambda_env(Symbol* sym, Environment* env) {
 Ast_Node* lookup_symbol_from_let_or_macro_env(Symbol* sym, Environment* env) {
     Ast_Node* result;
     do {
-	result = lookup_symbol_in_this_envt(sym, env);
-	if (result) return result;
-	if (env->type == Environment_Type_Lambda)
-	    break;
+        result = lookup_symbol_in_this_envt(sym, env);
+        if (result) return result;
+        if (env->type == Environment_Type_Lambda)
+            break;
 
-	env = env->parent;
+        env = env->parent;
     } while (env);
 
     if (env) {
-	do {
-	    if (env->type != Environment_Type_Lambda) {
-		result = lookup_symbol_in_this_envt(sym, env);
-		if (result) return result;
-	    }
-	    env = env->parent;
-	} while (env);
+        do {
+            if (env->type != Environment_Type_Lambda) {
+                result = lookup_symbol_in_this_envt(sym, env);
+                if (result) return result;
+            }
+            env = env->parent;
+        } while (env);
     }
 
     return nullptr;
@@ -123,29 +123,29 @@ Ast_Node* lookup_symbol(Symbol* sym, Environment* env) {
     Ast_Node* result;
     result = lookup_symbol_in_this_envt(sym, env);
     if (result)
-	return result;
+        return result;
 
     if (env->parent) {
-	if (env->type == Environment_Type_Lambda) {
-	    result = lookup_symbol_from_lambda_env(sym, env->parent);
-	} else {
-	    result = lookup_symbol_from_let_or_macro_env(sym, env->parent);
-	}
-
-	if (result)
-	    return result;
+        if (env->type == Environment_Type_Lambda) {
+            result = lookup_symbol_from_lambda_env(sym, env->parent);
+        } else {
+            result = lookup_symbol_from_let_or_macro_env(sym, env->parent);
+        }
+
+        if (result)
+            return result;
     }
 
     if (string_equal(sym->identifier, "nil")) {
-	return create_ast_node_nil();
+        return create_ast_node_nil();
     }
     if (string_equal(sym->identifier, "t")) {
-	return create_ast_node_t();
+        return create_ast_node_t();
     }
 
     result = create_ast_node_built_in_function(sym->identifier);
     if (result)
-	return result;
+        return result;
 
     create_error(Error_Type_Symbol_Not_Defined, create_ast_node_nil());
     /* printf("%s\n", sym->identifier); */
@@ -153,28 +153,28 @@ Ast_Node* lookup_symbol(Symbol* sym, Environment* env) {
 }
 void print_indent(int indent) {
     for (int i = 0; i < indent; ++i) {
-	printf(" ");
+        printf(" ");
     }
 }
 
 void print_environment_indent(Environment* env, int indent) {
     for (int i = 0; i < env->next_index; ++i) {
-	print_indent(indent);
-	printf("%s -> ", env->keys[i]);
-	print(env->values[i]);
-	printf("\n");
+        print_indent(indent);
+        printf("%s -> ", env->keys[i]);
+        print(env->values[i]);
+        printf("\n");
     }
     if (env->parent) {
-	print_indent(indent);
-	printf("parent");
-	if (env->parent->type == Environment_Type_Lambda)
-	    printf(" (lambda)");
-	else if (env->parent->type == Environment_Type_Macro)
-	    printf(" (macro)");
-	else if (env->parent->type == Environment_Type_Let)
-	    printf(" (let)");
-	printf(":\n");
-	print_environment_indent(env->parent, indent+4);
+        print_indent(indent);
+        printf("parent");
+        if (env->parent->type == Environment_Type_Lambda)
+            printf(" (lambda)");
+        else if (env->parent->type == Environment_Type_Macro)
+            printf(" (macro)");
+        else if (env->parent->type == Environment_Type_Let)
+            printf(" (let)");
+        printf(":\n");
+        print_environment_indent(env->parent, indent+4);
     }
 }
 
diff --git a/src/eval.c b/src/eval.c
index bb3d80a..97b0089 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -77,7 +77,8 @@ Ast_Node* apply_arguments_to_function(Ast_Node* arguments, Function* function, E
         // if not set it and then add it to the array list
         define_symbol(
             create_ast_node_symbol(arguments->value.pair->first->value.keyword->identifier),
-            arguments->value.pair->rest->value.pair->first, new_env);
+            arguments->value.pair->rest->value.pair->first,
+            new_env);
 
         append_to_String_array_list(read_in_keywords, arguments->value.pair->first->value.keyword->identifier);
 
@@ -115,7 +116,7 @@ Ast_Node* apply_arguments_to_function(Ast_Node* arguments, Function* function, E
             if (!was_set) {
                 define_symbol(
                     create_ast_node_symbol(defined_keyword),
-                    function->keyword_arguments->values->data[i], new_env);
+                    copy_ast_node(function->keyword_arguments->values->data[i]), new_env);
             }
         }
     }
@@ -151,7 +152,6 @@ Ast_Node* apply_arguments_to_function(Ast_Node* arguments, Function* function, E
             result = eval_expr(function->body, new_env);
         }
 
-        free(new_env);
         return result;
     }
 }
@@ -456,8 +456,18 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
                         printf("Keyword:     {");
                         if (fun->value.function->keyword_arguments->next_index != 0) {
                             printf("%s", fun->value.function->keyword_arguments->identifiers[0]);
+                            if (fun->value.function->keyword_arguments->values->data[0]) {
+                                printf(" (");
+                                print(fun->value.function->keyword_arguments->values->data[0]);
+                                printf(")");
+                            }
                             for (int i = 1; i < fun->value.function->keyword_arguments->next_index; ++i) {
                                 printf(", %s", fun->value.function->keyword_arguments->identifiers[i]);
+                                if (fun->value.function->keyword_arguments->values->data[i]) {
+                                    printf(" (");
+                                    print(fun->value.function->keyword_arguments->values->data[i]);
+                                    printf(")");
+                                }
                             }
                         }
                         printf("}\n");
@@ -808,6 +818,22 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
                 *target = *source;
                 return target;
             }
+            case Built_In_Copy: {
+                if (arguments_length != 1)
+                    report_error(Error_Type_Wrong_Number_Of_Arguments);
+
+                if (evaluated_arguments->value.pair->first->type == Ast_Node_Type_Nil ||
+                    evaluated_arguments->value.pair->first->type == Ast_Node_Type_Keyword)
+                {
+                    report_error(Error_Type_Type_Missmatch);
+                }
+
+                Ast_Node* target = new(Ast_Node);
+                Ast_Node* source = evaluated_arguments->value.pair->first;
+
+                *target = *source;
+                return target;
+            }
             case Built_In_Load: {
                 if (arguments_length != 1)
                     report_error(Error_Type_Wrong_Number_Of_Arguments);