code: protected function getPermissionGroupIndex($groups)
      {
          $index = [];
  
          foreach ($groups as $key => $presence) {
  
              $permissions = $presence->permissions();
  
              if ( ! $permissions) {
                  continue;
              }
  
              if ( ! is_array($permissions)) {
                  $permissions = [ $permissions ];
              }
  
              foreach ($permissions as $permission) {
                  $index[ $permission ] = $key;
              }
          }
  
          return $index;
      }
docs: Returns an index for the group for each permission.
  
  @param AclPresenceInterface[] $groups
  @return string[]     associative: keyed by permission slug