input: Write a function to sum elements of a list, ignoring negative numbers.
  assert sum_without_negatives([-4, 1, 6, 2, -9, 5]) == 14
  assert sum_without_negatives([-1, -2, -3, -4]) == 0
  assert sum_without_negatives([0, 0, 0, 0]) == 0
  assert sum_without_negatives([10, -10, 20, -20, 30, -30]) == 60
output: def sum_without_negatives(lst):
      return sum(x for x in lst if x >= 0)