A developer needs to compute a discounted price for each product returned by ad-hoc SQL queries. The discount rate is a fixed business formula that depends only on values already stored in each document. The logic must be reusable inside the WHERE and SELECT clauses of many different queries, must not require a separate write operation, and should execute within the scope of a single logical partition without extra round-trips. Which server-side construct should the developer implement in Azure Cosmos DB?