What is the test for establishing that a written agreement supersedes all previous understandings?

"Appellant urges the application of the general rule articulated by this court in Nuquist v. Bauscher, 'Where preliminary negotiations are consummated by written agreement, the writing supersedes all previous understandings and the intent of the parties must be ascertained from the writing.' This rule governs only when the integrated character of the writing is established. Whether a particular subject of negotiation is embodied in the writing depends on the intent of the parties, revealed by their conduct and language, and by the surrounding circumstances. Mere existence of a document does not establish integration. (In a footnote the court said: . . . An earlier view, expressed in Thompson v. Libby, 34 Minn. 374, 26 N.W. 1, 2 (1885), that the only criterion of the completeness of a written contract is the writing itself is no longer authoritative. . . .)"