docs: @Nonnull
    @ReturnsMutableCopy
    public ICommonsList <String> getAllUserGroupIDsWithAssignedUser (@Nullable final String sUserID)
    {
      if (StringHelper.hasNoText (sUserID))
        return new CommonsArrayList <> ();
  
      return getAllMapped (aUserGroup -> aUserGroup.containsUserID (sUserID), aUserGroup -> aUserGroup.getID ());
    }