input: Write a function to execute a valid insertion sort on a given list of integers.
  
  
  def insertion_sort(arr):
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr
  
  assert insertion_sort([5, 2, 9, 1, 5, 6]) == [1, 2, 5, 5, 6, 9]
  assert insertion_sort([3, 0, 2, 5, -1, 4, 1]) == [-1, 0, 1, 2, 3, 4, 5]
  assert insertion_sort([]) == []
  assert insertion_sort([1]) == [1]
  assert insertion_sort([100, 2, 30, 11]) == [2, 11, 30, 100]
  
output: def insertion_sort(arr):
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr