(1) Whenever the terms of an agreement have been reduced to writing by the parties, it is to be considered as containing all those terms. 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) when a mistake or imperfection of the writing is put in issue by the pleadings;
(b) when the validity of the agreement is the fact in dispute.
(2) This section does not exclude other evidence of the circumstances under which the agreement was made or to which it relates, as described in 1-4-102, or other evidence to explain an extrinsic ambiguity or to establish illegality or fraud.
(3) The term "agreement", for the purposes of this section, includes deeds and wills as well as contracts between parties.