70-20-202. Extrinsic evidence not to be considered in construction of deed -- exceptions. (1) When the terms of an agreement have been reduced to writing by the parties, it is to be considered as containing all those terms, and therefore there can be between the parties and their representatives or successors in interest no evidence of the terms of the agreement other than the contents of the writing except in the following cases:

(a) where a mistake or imperfection of the writing is put in issue by the pleadings;

(b) where the validity of the agreement is the fact in dispute.

(2) But this section does not exclude other evidence of the circumstances under which the agreement was made or to which it relates, as defined in 1-4-102, or to explain an extrinsic ambiguity or to establish illegality or fraud.

(3) The term "agreement" includes deeds and wills as well as contracts between parties.