input: Write a python function to sum even integers in a list.
  assert sum_even_integers([1, 2, 3, 4, 5, 6]) == 12
  assert sum_even_integers([10, 20, 30, 40, 50]) == 150
  assert sum_even_integers([1, 3, 5, 7, 9]) == 0
output: def sum_even_integers(lst):
      return sum(num for num in lst if num % 2 == 0)