The following is borrowed from Judge Carroll’s recent August 27, 2015 unpublished opinion on a claims objection which can be found here.
In law school, we learned that if a written instrument is valid, complete and unambiguous, extrinsic evidence is not admissible to vary, add to, or contradict the terms of the instrument. This is called the parol evidence rule. The exception to this rule is if there is an allegation of fraud, accident or mistake.