input: def insertion_sort(list):
      """ Implement the insertion sort algorithm in Python.
      >>> insertion_sort([4, 2, 1, 3])
      [1, 2, 3, 4]
      >>> insertion_sort([7, 5, 8, 6])
      [5, 6, 7, 8]
      >>> insertion_sort([])
      []
      >>> insertion_sort([1])
      [1]
      """
output:     for i in range(1, len(list)):
          key = list[i]
          j = i - 1
          while j >= 0 and key < list[j]:
              list[j + 1] = list[j]
              j -= 1
          list[j + 1] = key
      return list