Can an oral representation to a guarantor change the terms of the written guarantee?

I do not think that Hawrish v. Bank of Montreal, supra, which deals only with an allegation of a representation to a guarantor contrary to the express terms of the written guarantee obligation, is contrary to the conclusion that I have reached that, where there is an overall umbrella loan agreement which has been established by evidence to be a collateral agreement to the guarantee, the oral agreement may indicate how the guarantee or other security document should be interpreted. A debenture or promissory note may be framed as being payable on demand, but the umbrella loan agreement may say that demand will not be made unless there is a default of a specified kind. So "demand" in the promissory note or in the debenture, and as between the maker and the payee, would be given an interpretation that was consistent with the umbrella loan agreement. The umbrella agreement, as a collateral agreement, comes within the collateral agreement exception to the parol evidence rule, so long as it is a true collateral oral agreement established by the evidence, and not merely a unilateral oral representation in derogation from the express terms of the document, as was the case in Hawrish.