diff --git a/tests/nodes.phpt b/tests/nodes.phpt
index 144a30b..fb7d3c1 100644
--- a/tests/nodes.phpt
+++ b/tests/nodes.phpt
@@ -7,6 +7,9 @@ namespace Dakujem\Test;
 use Dakujem\Oliva\Node;
 use Dakujem\Oliva\Simple\NodeBuilder;
 use Dakujem\Oliva\Simple\TreeWrapper;
+use Dakujem\Oliva\Tree;
+use Dakujem\Oliva\TreeNodeContract;
+use Exception;
 use Tester\Assert;
 
 require_once __DIR__ . '/setup.php';
@@ -81,3 +84,129 @@ require_once __DIR__ . '/setup.php';
     // pre-order
     Assert::same('FBADCEGIH', TreeTesterTool::flatten($root));
 })();
+
+
+// Test linking/unlinking of nodes via Tree utility.
+(function () {
+    $a = new Node('A');
+    $b = new Node('B');
+    $c = new Node('C');
+    $d = new Node('D');
+    $e = new Node('E');
+
+    Tree::linkChildren($a, [$b, $c]);
+    Tree::linkChildren($b, [$d, $e]);
+
+    Assert::same('ABDEC', TreeTesterTool::flatten($a));
+    assert::same($a, $b->root());
+    assert::same($a, $c->root());
+    assert::same($a, $d->root());
+    assert::same($a, $e->root());
+
+    Assert::true($a->hasChild(0));
+    Assert::true($a->hasChild(1));
+    Assert::true($a->hasChild($b));
+    Assert::true($a->hasChild($c));
+    Assert::false($a->hasChild($d));
+    Assert::false($a->hasChild(2));
+
+    $hasRun = false;
+    Assert::same($b, $e->parent());
+    Assert::same([$d, $e], $b->children());
+    Assert::same([], $d->children());
+    Tree::linkChildren($d, $e, function (TreeNodeContract $originalParent) use (&$hasRun, $b) {
+        $hasRun = true;
+        // B is the original parent of E
+        Assert::same($b, $originalParent);
+    });
+    Assert::true($hasRun);
+    Assert::same('ABDEC', TreeTesterTool::flatten($a));
+    Assert::same($d, $e->parent());
+    Assert::same([$d], $b->children());
+    Assert::same([$e], $d->children());
+    assert::same($a, $e->root());
+
+    Tree::unlinkChildren($a);
+    Assert::same('A', TreeTesterTool::flatten($a));
+    Assert::true($a->isRoot());
+    Assert::true($a->isLeaf());
+    Assert::same([], $a->children());
+    Assert::true($b->isRoot());
+    Assert::true($c->isRoot());
+
+    // When unlinking node that is already a root, null is returned.
+    // Otherwise, the original parent is returned.
+    Assert::same(null, Tree::unlink($b));
+    Tree::link($b, $a, 'my-key');
+    Assert::false($a->isLeaf());
+    Assert::false($b->isRoot());
+    Assert::true($a->hasChild($b));
+    Assert::true($a->hasChild('my-key'));
+    Assert::false($a->hasChild(0));
+    Assert::same($a, Tree::unlink($b));
+    Assert::true($b->isRoot());
+})();
+
+
+// A data node can be filled with any data.
+(function () {
+    $a = new Node('A');
+    Assert::same('A', $a->data());
+    $a->fill('foo');
+    Assert::same('foo', $a->data());
+    $a->fill(null);
+    Assert::same(null, $a->data());
+    $a->fill($object = new Exception());
+    Assert::same($object, $a->data());
+})();
+
+
+
+(function () {
+    $tree = Preset::wikiTree();
+    $raw = json_decode(json_encode($tree), true);
+    Assert::same([
+        'data' => 'F',
+        'children' => [
+            [
+                'data' => 'B',
+                'children' => [
+                    [
+                        'data' => 'A',
+                        'children' => [],
+                    ],
+                    [
+                        'data' => 'D',
+                        'children' => [
+                            [
+                                'data' => 'C',
+                                'children' => [],
+                            ],
+                            [
+                                'data' => 'E',
+                                'children' => [],
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+            [
+                'data' => 'G',
+                'children' => [
+                    [
+                        'data' => 'I',
+                        'children' => [
+                            [
+                                'data' => 'H',
+                                'children' => [],
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+        ],
+    ], $raw);
+})();
+
+
+
diff --git a/tests/setup.php b/tests/setup.php
index 372b381..5c2471b 100644
--- a/tests/setup.php
+++ b/tests/setup.php
@@ -66,6 +66,14 @@ final class Preset
     /**
      * Returns manually built tree from Wikipedia:
      * @link https://en.wikipedia.org/wiki/Tree_traversal
+     *
+     *                 F
+     *                 |
+     *         B ------+------ G
+     *         |               |
+     *     A --+-- D           I
+     *             |           |
+     *         C --+-- E       H
      */
     public static function wikiTree(): Node
     {