Article 51 of Civil Code

1 - If the contracting parties agree that the provisions of a model contract or model regulations shall govern their affairs, those provisions shall apply unless either party proves that, when the agreement between them was concluded, he was unaware of those provisions and had not been afforded the opportunity to become aware of them. 2 - If the provisions of the model contract or model regulations of which there was no knowledge are essential, the contract is void. If they are secondary, the judge shall settle the dispute concerning them in accordance with the nature of the transaction, prevailing custom and the requirements of justice.