The parties may exclude the application of this Convention or,subject to article 12, derogate from or vary the effect of any of its provisions. (Formation of the contract) Article 14 (1)A proposal for concluding a contract addressed to one or more specific persons constitutes an offer if it is sufficiently definite and indicates the intention of the offeror to be bound in case of acceptance.A proposal is sufficiently definite if it indicates the goods and expressly or implicitly fixes or makes provision for determining the quantity and the price. (2)A proposal other than one addressed to one or more specific persons is to be considered merely as an invitation to make offers,unless the contrary is clearly indicated by the person making the proposal. Article 15 (1)An offer becomes effective when it reaches the offeree. (2)An offer,even if it is irrevocable,may be withdrawn if the withdrawal reaches the offeree before or at the same time as the offer. Article 16 (1)Until a contract is concluded an offer may be revoked if the revocation reaches the offeree before he has dispatched an acceptance. (2)However,an offer cannot be revoked: (a)if it indicates,whether by stating a fixed time for acceptance or otherwise,that it is irrevocable;or (b)if it was reasonable for the offeree to rely on the offer as being irrevocable and the offeree has acted in reliance on the offer. Article 17 An offer,even if it is irrevocable,is terminated when a rejection reaches the offeror. Article 18 3.- 3 - The parties may exclude the application of this Convention or, subject to article 12, derogate from or vary the effect of any of its provisions. (Formation of the contract) Article 14 (1) A proposal for concluding a contract addressed to one or more specific persons constitutes an offer if it is sufficiently definite and indicates the intention of the offeror to be bound in case of acceptance. A proposal is sufficiently definite if it indicates the goods and expressly or implicitly fixes or makes provision for determining the quantity and the price. (2) A proposal other than one addressed to one or more specific persons is to be considered merely as an invitation to make offers, unless the contrary is clearly indicated by the person making the proposal. Article 15 (1) An offer becomes effective when it reaches the offeree. (2) An offer, even if it is irrevocable, may be withdrawn if the withdrawal reaches the offeree before or at the same time as the offer. Article 16 (1) Until a contract is concluded an offer may be revoked if the revocation reaches the offeree before he has dispatched an acceptance. (2) However, an offer cannot be revoked: (a) if it indicates, whether by stating a fixed time for acceptance or otherwise, that it is irrevocable; or (b) if it was reasonable for the offeree to rely on the offer as being irrevocable and the offeree has acted in reliance on the offer. Article 17 An offer, even if it is irrevocable, is terminated when a rejection reaches the offeror. Article 18