EDIT

   @Override

   public TaxonNode addChildTaxon(Taxon taxon, DescriptionElementSource source) {

       return addChildTaxon(taxon, this.childNodes.size(), source);

   }

    @Override

    public TaxonNode addChildTaxon(Taxon taxon, int index, DescriptionElementSource source) {

        Classification classification = CdmBase.deproxy(this.getClassification());

        taxon = HibernateProxyHelper.deproxy(taxon, Taxon.class);

        if (classification.isTaxonInTree(taxon)){

            throw new IllegalArgumentException(String.format("Taxon may not be in a classification twice: %s", taxon.getTitleCache()));

       }

       return addChildNode(new TaxonNode(taxon), index, source);

    }

    /**

     * Inserts the given taxon node in the list of children of <i>this</i> taxon node

     * at the given (index + 1) position. If the given index is out of bounds

     * an exception will arise.<BR>

     * Due to bidirectionality this method must also assign <i>this</i> taxon node

     * as the parent of the given child.

     *

     * @param   child   the taxon node to be added

     * @param   index   the integer indicating the position at which the child

     *                  should be added

     * @see             #getChildNodes()

     * @see             #addChildNode(TaxonNode, Reference, String, Synonym)

     * @see             #deleteChildNode(TaxonNode)

     * @see             #deleteChildNode(int)

     */

    @Override

    public TaxonNode addChildNode(TaxonNode child, int index, DescriptionElementSource source){

        if (index < 0 || index > childNodes.size() + 1){

            throw new IndexOutOfBoundsException("Wrong index: " + index);

        }

           // check if this node is a descendant of the childNode

        if(child.getParent() != this && child.isAncestor(this)){

            throw new IllegalAncestryException("New parent node is a descendant of the node to be moved.");

        }

        child.setParentTreeNode(this, index);

        child.setSource(source);

        return child;

    }