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1. This Convention does not affect any international instrument concluded before this Convention to which Contracting States are Parties and which contains provisions on matters governed by this Convention.
2. Any Contracting State may conclude with one or more Contracting States agreements, which contain provisions on matters governed by the Convention, with a view to improving the application of the Convention between or among themselves, provided that such agreements are consistent with the objects and purpose of the Convention and do not affect, in the relationship of such States with other Contracting States, the application of the provisions of the Convention. The States which have concluded such an agreement shall transmit a copy to the depositary of the Convention.
3. Paragraphs 1 and 2 shall also apply to reciprocity arrangements and to uniform laws based on special ties between the States concerned.
4. This Convention shall not affect the application of instruments of a Regional Economic Integration Organisation that is a Party to this Convention, adopted after the conclusion of the Convention, on matters governed by the Convention provided that such instruments do not affect, in the relationship of Member States of the Regional Economic Integration Organisation with other Contracting States, the application of the provisions of the Convention. As concerns the recognition or enforcement of decisions as between Member States of the Regional Economic Integration Organisation, the Convention shall not affect the rules of the Regional Economic Integration Organisation, whether adopted before or after the conclusion of the Convention.