input: Write a python function to sum even elements in a list.
  assert sum_even_elements([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 30
  assert sum_even_elements([10, 20, 45, 67, 84, 93]) == 114
  assert sum_even_elements([5, 7, 9, 8, 6, 4, 3]) == 18
output: def sum_even_elements(lst):
      return sum(x for x in lst if x % 2 == 0)