diff -r a10c3e127050 -r bb144d08cd44 src/cx/tree.h
--- a/src/cx/tree.h	Sun Oct 03 13:07:48 2021 +0200
+++ b/src/cx/tree.h	Sun Oct 03 14:06:57 2021 +0200
@@ -25,6 +25,14 @@
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */
+/**
+ * \file tree.h
+ * \brief Interface for tree implementations.
+ * \author Olaf Wintermann
+ * \author Mike Becker
+ * \version 3.0
+ * \copyright 2-Clause BSD License
+ */
 
 #ifndef UCX_TREE_H
 #define UCX_TREE_H
@@ -36,23 +44,93 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
- 
-void* cx_tree_last(void *node, ptrdiff_t loc_next);
-    
-int cx_tree_add_node(void *node, ptrdiff_t loc_parent, ptrdiff_t loc_prev, ptrdiff_t loc_next, void *new_node);
 
-int cx_tree_add_child_node(
-        void *parent,
-        ptrdiff_t loc_parent,
-        ptrdiff_t loc_prev,
-        ptrdiff_t loc_next,
-        void **children_begin,
-        void **children_end,
-        void *new_node);
+/**
+ * Adds a sibling to the current tree node.
+ *
+ * In case your struct does not have a \p prev or a \p parent pointer,
+ * specify a negative location. The location of the \p next pointer is
+ * mandatory.
+ *
+ * \attention Do not use this function to add siblings in a tree where the
+ * nodes store a pointer to the last sibling because that would not be modified by this function.
+ *
+ * \remark If yo do not provide a location to the parent pointer, a call to this function is
+ * effectively the same as a call to cx_linked_list_add().
+ *
+ * @param node a pointer to the node
+ * @param loc_prev the location of a \c prev pointer within your node struct
+ * @param loc_next the location of a \c next pointer within your node struct
+ * @param loc_parent the location of a \c parent pointer within your node struct
+ * @param new_node the new node that shall be added as a sibling
+ */
+void cx_tree_add_sibling(void *node,
+                         ptrdiff_t loc_prev, ptrdiff_t loc_next,
+                         ptrdiff_t loc_parent,
+                         void *new_node)
+__attribute__((__nonnull__));
+
+/**
+ * Adds a node to the list of children.
+ *
+ * \par Example with a full structure
+ * A full tree node structure may look like this:
+ * \code
+ * typedef struct MyTreeNode MyTreeNode;
+ * struct MyTreeNode {
+ *   MyTreeNode* parent;
+ *   MyTreeNode* first_child;
+ *   MyTreeNode* last_child;
+ *   MyTreeNode* prev_sibling;
+ *   MyTreeNode* next_sibling;
+ *   // ...contents...
+ * }
+ * \endcode
+ * Adding a new child to a node with the above structure can be performed with the following call:
+ * \code
+ * MyTreeNode *node, *child; // given
+ * cx_tree_add_child(&node->first_child, &node->last_child,
+ *                   offsetof(MyTreeNode, prev_sibling), offsetof(MyTreeNode, next_sibling),
+ *                   child, offsetof(MyTreeNode, parent), node);
+ * \endcode
+ *
+ * \par Example with a reduced structure
+ * The minimal reasonable structure with parent pointer looks like this:
+ * \code
+ * typedef struct MyTreeNode MyTreeNode;
+ * struct MyTreeNode {
+ *   MyTreeNode* parent;
+ *   MyTreeNode* children;
+ *   MyTreeNode* next_sibling;
+ *   // ...contents...
+ * }
+ * \endcode
+ * This simplifies the function call to:
+ * \code
+ * MyTreeNode *node, *child; // given
+ * cx_tree_add_child(&node->children, NULL, -1, offsetof(MyTreeNode, next_sibling),
+ *                   child, offsetof(MyTreeNode, parent), node);
+ * \endcode
+ *
+ * \remark If your tree structure does not possess a parent pointer, a call to this function is
+ * effectively the same as a call to cx_linked_list_add().
+ *
+ * @param children_begin a pointer to the begin node pointer (if your list has one)
+ * @param children_end a pointer to the end node pointer (if your list has one)
+ * @param loc_prev the location of a \c prev pointer within your node struct
+ * @param loc_next the location of a \c next pointer within your node struct
+ * @param new_node a pointer to the node that shall be appended
+ * @param loc_parent the location of a \c parent pointer within your node struct
+ * @param parent the parent node
+ */
+void cx_tree_add_child(void **children_begin, void **children_end,
+                       ptrdiff_t loc_prev, ptrdiff_t loc_next, void *new_node,
+                       ptrdiff_t loc_parent, void *parent)
+__attribute__((__nonnull__ (5)));
 
 
 #ifdef __cplusplus
-}
+} /* extern "C" */
 #endif
 
 #endif /* UCX_TREE_H */