“it is necessary to show either (1) that the document fails to give effect to a prior concluded contract or (2) that, when they executed the document, the parties had a common intention in respect of a particular matter which, by mistake, the document did not accurately record. In the latter case it is necessary to show not only that each party to the contract had the same actual intention with regard to the relevant matter, but also that there was an outward expression of accord meaning that, as a result of communication between them, the parties understood each other to share that intention”
“the courts should not ordinarily infer that a contracting party has given up rights that the law has conferred upon him to an extent greater than the contractual terms indicate he has chosen to do”