docs: def incrementor(start=0, step=1):
    """Returns a function that first returns the start value, and returns previous value + step on
    each subsequent call.
    """
    def fxn(_):
      """Returns the next value in the sequnce defined by [start::step)"""
      nonlocal start
      rval = start
      start += step
      return rval
    return fxn