code: @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 ());
    }
docs: Get a collection of all user group IDs to which a certain user is assigned
  to.
  
  @param sUserID
  The user ID to search
  @return A non-<code>null</code>but may be empty collection with all
  matching user group IDs.