Is parol evidence admissible to vary or modify the express terms of a written contract?

See, to the same effect, the ruling in Lacasse v Prairie Pulse Inc., 2005 SKCA 80 where it was held at para. 3: 3. Generally speaking the rule is that parol evidence is not admissible to vary or modify the express terms of a written contract. If the parties have reduced the terms of their contract to writing in clear and unambiguous manner, the express terms define the mutual obligations of the parties. Oral evidence in contradiction of the terms is inadmissible: Hawrish v. Bank of Montreal, 1969 CanLII 2 (SCC), [1969] S.C.R. 515.