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

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.