type Result = 'OK' | 'Falsified' | 'Exception';

type TestResult<A> = {
  result: Result,
  seed: number,
  counterexample: A | null
};

type Predicate<A> = (a: A) => boolean;

type Gen<A> = (s: number) => (() => A);

type Shrinker<A> = (a: A) => [A];

function property<A>(seed: number,
  predicate: Predicate<A>,
  generator: Gen<A>,
  shrinker: Shrinker<A>): TestResult<A> {
  return {result: 'OK', seed, counterexample: null};
}