input: Write a python function to calculate the sum of even numbers in a sequence.
  assert sum_even_numbers([1, 2, 3, 4, 5, 6]) == 12
  assert sum_even_numbers([10, 21, 32, 43, 54]) == 96
  assert sum_even_numbers([7, 13, 19]) == 0
output: def sum_even_numbers(sequence):
      return sum(num for num in sequence if num % 2 == 0)