lulu-berlin

// Seq.unfold (https://msdn.microsoft.com/en-us/library/ee340363.aspx)
// works with a generator function that gets a state and returns
//   Some (element, state) to generate another iterator, or
//   None to stop the iteration.
// The problem is that tuples are allocated every single iteration.
//
// The solution below uses instead a generator function that gets
// a state and an iterator function. The iterator function can be called
// to yield a new element and set a new state for the next iterator. If the
// generator function returns a unit, i.e. (), the iteration stops.