[Haskell-cafe] QuickCheck: Why are these instances of Testable?

[Paul Brenman]

Why does QuickCheck (2.6) list the following as instances of class Testable?

Testable Result
Testable Prop
Testable prop => Testable (Gen prop)

The Hughes/Claessen paper "QuickCheck: A Lightweight Tool for Random
Testing of Haskell Programs" mentions nesting property combinators on page
6, but I didn't understand this reference.
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