code: def alternate(*iterables):
      
      items = []
      for tup in zip(*iterables):
          items.extend([item for item in tup])
      return items
docs: [a[0], b[0], ... , a[1], b[1], ..., a[n], b[n] ...]
      >>> alternate([1,4], [2,5], [3,6])
      [1, 2, 3, 4, 5, 6]