code: public static function normalizeIdentities($identities)
      {
          if (is_array($identities)) {
              $identities = array_map([__CLASS__, 'normalizeIdentity'], $identities);
          }
  
          if (null === $identities) {
              return [];
          }
  
          if (is_string($identities)) {
              $identities = [self::normalizeIdentity($identities)];
          }
  
          if (!is_array($identities) && $identities instanceof IdentityInterface) {
              $identities = [$identities];
          }
  
          return $identities;
      }
docs: @param IdentityInterface[]|string $identities
  
  @return IdentityInterface[]