anchor.diff:  
       ///The workflow type for generators.
       type GenBuilder internal() =
  -        member __.Return(a) : Gen<_> = ``return`` a
  +        member __.Return(a) : Gen<_> = result a
           member __.Bind(m, k) : Gen<_> = bind m k                            
           member __.Delay(f) : Gen<_> = delay f
           member __.Combine(m1, m2) = bind m1 (fun () -> m2)
  
positive.code:       if p() then
          bind m (fun _ -> doWhile p m)
        else
          ``return`` ()
  
      let private tryFinally (Gen m) handler = 
          Gen (fun n r -> try m n r finally handler ())
  
negative.code: [<AutoOpen>]
  module GenBuilder =
  
      let private result x = Gen (fun _ _ -> x)
  
      let private bind ((Gen m) : Gen<_>) (k : _ -> Gen<_>) : Gen<_> = 
          Gen (fun n r0 -> let r1,r2 = split r0
  