diff --git a/src/Element/RuleList.php b/src/Element/RuleList.php
index dfdfb9a..9593133 100755
--- a/src/Element/RuleList.php
+++ b/src/Element/RuleList.php
@@ -43,12 +43,14 @@ public function __get($name) {
 
             return $this->ast->children ?? [];
         }
+
+		return null;
     }
 
     /**
      * @inheritDoc
      */
-    public function addComment($value)
+    public function addComment(string $value): Comment
     {
 
         $comment = new Comment();
@@ -57,10 +59,11 @@ public function addComment($value)
         return $this->append($comment);
     }
 
-    /**
-     * @inheritDoc
-     */
-    public function computeShortHand() {
+	/**
+	 * @inheritDoc
+	 * @throws \Exception
+	 */
+    public function computeShortHand(): RuleListInterface {
 
        foreach (new NodeWalker($this) as $event => $node) {
 //		   ->
@@ -108,7 +111,7 @@ public function computeShortHand() {
     /**
      * @inheritDoc
      */
-    public function hasChildren()
+    public function hasChildren(): bool
     {
 
         return isset($this->ast->children) && count($this->ast->children) > 0;
@@ -117,7 +120,7 @@ public function hasChildren()
     /**
      * @inheritDoc
      */
-    public function removeChildren()
+    public function removeChildren(): ElementInterface
     {
 
         if (!empty($this->ast->isLeaf)) {
@@ -143,13 +146,13 @@ public function removeChildren()
     /**
      * @inheritDoc
      */
-    public function getChildren()
+    public function getChildren(): array
     {
 
         return $this->ast->children ?? [];
     }
 
-    public function setChildren(array $elements) {
+    public function setChildren(array $elements): RuleListInterface {
 
         if (!empty($this->ast->children)) {
 
@@ -194,14 +197,14 @@ public function support(ElementInterface $child): bool
     /**
      * @inheritDoc
      */
-    public function append(ElementInterface ...$elements)
+    public function append(ElementInterface ...$elements): ElementInterface|array
     {
 
         foreach ($elements as $element) {
 
             if (!$this->support($element)) {
 
-                throw new InvalidArgumentException(sprintf('%s: invalid child of type %s', $this->type, $element->type), 400);
+                throw new InvalidArgumentException(sprintf('%s: invalid child of type %s', $this->ast->type, $element->ast->type), 400);
             }
 
             if ($element instanceof Stylesheet) {
@@ -229,7 +232,7 @@ public function append(ElementInterface ...$elements)
     /**
      * @inheritDoc
      */
-    public function appendCss($css) {
+    public function appendCss(string $css): RuleListInterface {
 
         $this->append((new Parser($css))->parse());
 
@@ -239,7 +242,7 @@ public function appendCss($css) {
     /**
      * @inheritDoc
      */
-    public function insert(ElementInterface $element, $position)
+    public function insert(ElementInterface $element, int $position): ElementInterface
     {
         if (!$this->support($element) || $position < 0) {
 
@@ -268,8 +271,8 @@ public function insert(ElementInterface $element, $position)
     /**
      * @inheritDoc
      */
-    public function remove(ElementInterface $element)
-    {
+    public function remove(ElementInterface $element): ElementInterface
+	{
 
         if ($element->getParent() === $this) {
 
@@ -289,8 +292,8 @@ public function remove(ElementInterface $element)
      * return an iterator of child nodes
      * @return ArrayIterator|Traversable
      */
-    public function getIterator()
-    {
+    public function getIterator(): Traversable|ArrayIterator
+	{
 
         return new ArrayIterator($this->ast->children ?? []);
     }
diff --git a/src/Interfaces/RuleListInterface.php b/src/Interfaces/RuleListInterface.php
index 4248f26..a0a34fc 100755
--- a/src/Interfaces/RuleListInterface.php
+++ b/src/Interfaces/RuleListInterface.php
@@ -4,6 +4,7 @@
 
 use Exception;
 use IteratorAggregate;
+use TBela\CSS\Element\Comment;
 
 /**
  * Interface implemented by rules containers
@@ -18,12 +19,12 @@ interface RuleListInterface extends ElementInterface, IteratorAggregate {
      * return true if the node has children
      * @return bool
      */
-    public function hasChildren();
+    public function hasChildren(): bool;
     /**
      * return child nodes
      * @return array
      */
-    public function getChildren();
+    public function getChildren(): array;
 
     /**
      * append child node
@@ -31,15 +32,15 @@ public function getChildren();
      * @return ElementInterface
      * @throws Exception
      */
-    public function setChildren(array $elements);
+    public function setChildren(array $elements): ElementInterface;
 
     /**
      * Add a comment node
      * @param string $value
-     * @return Element\Comment
+     * @return Comment
      * @throws Exception
      */
-    public function addComment($value);
+    public function addComment(string $value): Comment;
 
     /**
      * check if this node accepts element as a child
@@ -51,10 +52,10 @@ public function support(ElementInterface $child): bool;
     /**
      * append child nodes
      * @param ElementInterface|ElementInterface[] $elements
-     * @return ElementInterface
+     * @return ElementInterface|ElementInterface[]
      * @throws Exception
      */
-    public function append(ElementInterface ...$elements);
+    public function append(ElementInterface ...$elements): ElementInterface|array;
 
     /**
      * append css text to this node
@@ -62,7 +63,7 @@ public function append(ElementInterface ...$elements);
      * @return ElementInterface
      * @throws Exception
      */
-    public function appendCss($css);
+    public function appendCss(string $css): ElementInterface;
 
     /**
      * insert a child node at the specified position
@@ -71,23 +72,23 @@ public function appendCss($css);
      * @return ElementInterface
      * @throws Exception
      */
-    public function insert(ElementInterface $element, $position);
+    public function insert(ElementInterface $element, int $position): ElementInterface;
 
     /**
      * remove a child node from its parent
      * @param ElementInterface $element
      * @return ElementInterface
      */
-    public function remove(ElementInterface $element);
+    public function remove(ElementInterface $element): ElementInterface;
 
     /**
      * Remove all children
      * @return ElementInterface
      */
-    public function removeChildren();
+    public function removeChildren(): ElementInterface;
 
     /**
      * @return RuleListInterface
      */
-    public function computeShortHand();
+    public function computeShortHand(): RuleListInterface;
 }
\ No newline at end of file