n9 "It is 'solely a judicial function to interpret a written instrument unless the interpretation turns upon the credibility of extrinsic evidence.' " (U.S. Leasing Corp. v. DuPont (1968) 69 Cal.2d 275, 284 [70 Cal.Rptr. 393, 444 P.2d 65], quoting Parsons v. Bristol Dev. Co. (1965) 62 Cal.2d 861, 865 [44 Cal.Rptr. 767, 402 P.2d 839].) Here, defendants have not offered any extrinsic evidence to aid in interpreting the waiver provisions in the guaranty agreements. Consequently, we may resolve this issue as a question of law.