input: Write a function to select negative elements from a list.
  assert select_negative_elements([3,4,1,-6,9,10,-2,-8,-4])==[-6, -2, -8, -4]
  assert select_negative_elements([10,20,45,67,-84,93])==[-84]
  assert select_negative_elements([5,7,9,8,6,4,3])==[]
output: def select_negative_elements(lst):
      return [x for x in lst if x < 0]