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Strategies for properties: metamorphic properties
Sometimes you need to test something that doesn't have a right answer. Metamorphic properties let you test a system against itself. If you modify the input, you should see a modification of the output.
Behind the scenes: size
It's important to know about size. Size controls the range of values of a generator. It changes over the length of a run of a property. The built-in generators have an intuitive notion of size, and you can control the size yourself using three functions from the generators namespace.