input: Write a Python function to identify the second minimum value in a list.
  
  assert second_minimum([1, 2, -8, -2, 0, -2]) == -2
  
  assert second_minimum([1, 1, -0.5, 0, 2, -2, -2]) == -0.5
  
  assert second_minimum([2, 2]) == None
output: def second_minimum(lst):
      if len(lst) < 2:
          return None
      first, second = float('inf'), float('inf')
      for num in lst:
          if num < first:
              first, second = num, first
          elif first < num < second:
              second = num
      return second if second != float('inf') else None