anchor.diff:  # QCheck_runner.run_tests [test];;
   random seed: 415927808
   success (ran 1 tests)
  -- : int = 0  
  +- : int = 0
   ----
   
   
  
positive.code: Other combinators include monadic abstraction, lifting functions,
  generation of lists, arrays, and a choice function.
  
  
negative.code: (* generate a few trees, just to check what they look like: *)
  QCheck.Gen.generate ~n:20 tree_gen;;
  
  let arbitrary_tree =
    let open QCheck.Iter in
    let rec shrink_tree = function
      | Leaf i -> QCheck.Shrink.int i >|= leaf
  