diff -r 4cea6e50175b -r ec92b4ed23aa src/tree.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/tree.c	Sun Sep 26 14:45:51 2021 +0200
@@ -0,0 +1,70 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2021 Mike Becker, Olaf Wintermann All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "cx/tree.h"
+
+#define CX_TR_PTR(cur, off) ((void**)(((char*)cur)+off))
+
+void* cx_tree_last(void *node, ptrdiff_t loc_next) {
+    void *last;
+    do {
+        last = node;
+    } while ((node = *CX_TR_PTR(node, loc_next)) != NULL);
+    return last;
+}
+
+int cx_tree_add_node(void *node, ptrdiff_t loc_parent, ptrdiff_t loc_prev, ptrdiff_t loc_next, void *new_node) {
+    void *last = cx_tree_last(node, loc_next);
+    if(!last)
+        return 1;
+    
+    // next pointer must be present
+    *CX_TR_PTR(last, loc_next) = new_node;
+    
+    // optional fields
+    if(loc_parent >= 0) {
+        *CX_TR_PTR(new_node, loc_parent) = *CX_TR_PTR(node, loc_parent);
+    }
+    if(loc_prev >= 0) {
+        *CX_TR_PTR(new_node, loc_prev) = last;
+    }
+    
+    return 0;
+}
+
+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)
+{
+    return 1;
+}