 Likewise, where there are not two collateral agreements but one composite contract, a portion of which is written and the remainder oral, evidence may be led to prove the supplemental oral portion, provided it is clear that the parties did not intend the written portion to be the exclusive memorial of the entire agreement.In such a case, termed a “partial integration,” the integration rule merely prevents the admission of extrinsic evidence to contradict or vary the written portion of the agreement.The court may hear evidence of surrounding circumstances, including the negotiations of the parties, to determine whether they intended the written agreement to be an integration of their whole transaction or merely a partial integration.