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function mymodule_menu() {
  $items['mymodule/%'] = array(
    'title' => 'My module callback',
    'type' => MENU_CALLBACK,
    'page callback' => 'mymodule_callback',
    'page arguments' => array(1),
    'access callback' => TRUE,
  );
}

function mymodule_node_view($node, $build_mode = 'full') {
  if ($node->type == 'mynodetype' && $build_mode == 'full') {
      $node->content['mylink'] = array(
        '#type' => 'link',
        '#title' => t('Do the magic'),
        '#href' => 'mymodule/'.$node->nid,
        '#prefix' => '<div id="mylink">',
        '#suffix' => '</div><div id="mylink-ajax-result"></div>',
        '#ajax' => array(
          'effect' => 'fade',
        ),
      );
  }
}

function mymodule_callback($nid) {
  $output = 'Yeah';
  $commands = array();
  $commands[] = ajax_command_replace('#mylink-ajax-result', $output);
  $page = array('#type' => 'ajax', '#commands' => $commands);
  ajax_deliver($page);
}

$form['main_fieldset'] = array(
  '#type' => 'fieldset',
  '#title' => 'my fieldset'
);
$form['main_fieldset']['mytabs'] = array(
  '#type' => 'vertical_tabs',
);
$form['main_fieldset']['other'] = array(
  '#type' => 'fieldset',
  '#title' => 'my fieldset',
  '#group' => 'mytabs'
);

Here are some pieces of code to play with nodes and “relation” module.

function relation_new_child_node_view($node, $build_mode = 'full') {
  if ($build_mode == 'full') {
    // get available relations
    $relation_types = relation_get_available_types('node', $node->type, 'target');
    if ($relation_types) {
      $content = array();
      foreach ($relation_types as $relation_type) {
        // get available node types
        $nodes_types = array();
        $items = array();
        foreach ($relation_type->source_bundles as $source_bundle) {
          $values = explode(':', $source_bundle);
          $nodes_types[$values[1]] = node_type_get_name($values[1]);
          $items[] = l(
            t('Create a new !type', array('!type' => $nodes_types[$values[1]])),
            'node/add/' . $values[1],
            array(
              'query' => array(
              'destination' => 'node/' . $node->nid,
                'rt' => $relation_type->relation_type,
                'nid' => $node->nid
              )
            )
          );
        }
        if (!empty($items)) {
          $content['relation_links'] = array(
            '#markup' => theme_item_list(array(
              'items' => $items,
              'title' => t('Create related content'),
              'type' => 'ul',
              'attributes' => array()
              ))
          );
        }
        // get related content
        $query = relation_query('node', $node->nid);
        $query->entityCondition('bundle', $relation_type->relation_type);
        $results = $query->execute();
        if ($results) {
          // get the related nodes
          $related_nodes = array();
          foreach ($results as $result) {
            $relation = relation_load($result->rid);
            $entities = field_get_items('relation', $relation, 'endpoints');
            $related_nodes[] = node_load((int)$entities[0]['entity_id']);
          }
          if (!empty($related_nodes)) {
            $nodes_types = array_unique($nodes_types);
            foreach ($nodes_types as $node_type => $node_type_name) {
              $items = array();
              foreach ($related_nodes as $related_node) {
                if ($related_node->type == $node_type) {
                  $items[] = l($related_node->title, 'node/' . $related_node->nid, array('attributes' => array('title' => $related_node->title)));
                }
              }
              if (!empty($items)) {
                $content['related_links'][$node_type] = array(
                  '#markup' => theme_item_list(array(
                    'items' => $items,
                    'title' => $node_type_name,
                    'type' => 'ul',
                    'attributes' => array()
                    ))
                );
              }
            }
          }
        }
      }
      if (!empty($content['related_links'])) {
        $node->content[] = $content['related_links'];
      }
      if (!empty($content['relation_links'])) {
        $node->content[] = $content['relation_links'];
      }
    }
  }
}

function relation_new_child_node_insert($node) {
  dsm('passed');
  if ((isset($_GET['rt']) && !empty($_GET['rt'])) && (isset($_GET['nid']) && !empty($_GET['nid']))) {
    $endpoints = array();
    $endpoints[] = array('entity_type' => 'node', 'entity_id' => $node->nid);
    $endpoints[] = array('entity_type' => 'node', 'entity_id' => $_GET['nid']);

    $new_relation = relation_create($_GET['rt'], $endpoints);

    if ($rid = relation_save($new_relation)) {
      drupal_set_message(t('Relation created'), 'status', FALSE);
    }
    else {
      drupal_set_message(t('Creating relation failed'), 'warning', FALSE);
    }
  }
}

sudo apt install php-gd
sudo apt install php-mbstring
composer create-project drupal/recommended-project:8.x d8test