input: Write a function to sum all even numbers in a list.
  assert sum_even_elems([1, 2, 3, 4, 5, 6]) == 12
  assert sum_even_elems([10, 21, 30, 43, 50]) == 90
  assert sum_even_elems([7, 9, 11, 14]) == 14
output: def sum_even_elems(lst):
      return sum(x for x in lst if x % 2 == 0)