EDIT

/**

 * Fetches the list of visible annotations for each of the cdm entities and returns the footnote keys for them.

 *

 * The footnotes are passed to the FootnoteManager in order to store the annotations and to create the footnote keys.

 *

 * @see cdm_fetch_visible_annotations()

 *

 * @param array $cdm_entities

 *   An array of CdmBase instances.

 * @param $footnote_list_key_suggestion string

 *    optional parameter. If this parameter is left empty (null, 0, "") the footnote key will be determined be set to

 *    RenderHints::getFootnoteListKey().'-annotations' otherwise the supplied key will be used.

 *

 * @return array of footnote keys

 */

function cdm_entities_annotations_footnotekeys(array $cdm_entities, $footnote_list_key_suggestion = NULL) {

  $foot_note_keys = [];

  foreach ($cdm_entities as $cdm_entity) {

        $foot_note_keys = array_merge($foot_note_keys, cdm_entity_annotations_as_footnotekeys($cdm_entity,$footnote_list_key_suggestion));

  }

  return $foot_note_keys;

}

/**

 * Fetches the list of visible annotations for the cdm entity or for the comparable

 * object and returns the footnote keys.

 *

 * The footnotes are passed to the FootnoteManager in order to store the

 * annotations and to create the footnote keys.

 * @param stdClass $cdm_entity

 *   A single CdmBase instance ore comparable object.

 * @param $footnote_list_key_suggestion string

 *    optional parameter. If this parameter is left empty (null, 0, "") the footnote key will be determined be set to

 *    RenderHints::getFootnoteListKey().'-annotations' otherwise the supplied key will be used.

 * @return array of footnote keys

 *

 * @see cdm_fetch_visible_annotations()

 */

function cdm_entity_annotations_as_footnotekeys(stdClass $cdm_entity, $footnote_list_key_suggestion = NULL) {

  $foot_note_keys = [];

  // Getting the key for the footnotemanager.

  if (isset($footnote_list_key_suggestion)) {

    $footnote_list_key = $footnote_list_key_suggestion;

  } else {

    $footnote_list_key = RenderHints::getFootnoteListKey() . '-annotations';

  }

  // Adding the footnotes keys.

  $annotations = cdm_fetch_visible_annotations($cdm_entity);

  if (is_array($annotations)) {

    foreach ($annotations as $annotation) {

      $foot_note_keys[] = FootnoteManager::addNewFootnote($footnote_list_key, $annotation->text);

    }

  }

  return $foot_note_keys;

}