The Codelfa requirement that surrounding circumstances must be mutually known to both parties (by direct proof or inference from notoriety) to be admissible as an aid to contractual construction has not been relaxed by the reference in Maggbury v Hafele to background knowledge 'reasonably available' to the parties. Constructive or imputed knowledge is insufficient. Where a product can only be acquired as part of a package, its 'retail price' ordinarily refers to the incremental price specifically attributable to that product, not the cost of the entire package. A document exhibited to a contract forms part of the contract and is available for construction purposes without needing to satisfy the requirements for admissibility of extrinsic evidence.
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