code: @Nonnull
    @ReturnsMutableCopy
    public static IMultiMapListBased <IParticipantIdentifier, PDStoredBusinessEntity> getGroupedByParticipantID (@Nonnull final Iterable <PDStoredBusinessEntity> aDocs)
    {
      final MultiLinkedHashMapArrayListBased <IParticipantIdentifier, PDStoredBusinessEntity> ret = new MultiLinkedHashMapArrayListBased <> ();
      for (final PDStoredBusinessEntity aDoc : aDocs)
        ret.putSingle (aDoc.getParticipantID (), aDoc);
      return ret;
    }
docs: Group the passed document list by participant ID
  
  @param aDocs
  The document list to group.
  @return A non-<code>null</code> ordered map with the results. Order is like
  the order of the input list.