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Equivalence Partitioning: Splitting Inputs into Classes, Valid and Invalid Partitions, Coverage and Turning Them into xUnit Theories

Equivalence partitioning splits an input into partitions the code should treat the same way and covers each with one test, while every invalid partition gets a test of its own. In Bookstore's order validator, tests combining two invalid values hid a missing guest payment rule; nine tests derived from the partition table caught it and an undefined enum value bug. Each Choice coverage, output partitions and TheoryData.