input: Write a python function to sum the integers in a list.
  assert sum_integer([5, 2, 8, 4, 23]) == 42
  assert sum_integer([1, 2, 3, 4, 5]) == 15
  assert sum_integer([10, -2, 3, 7]) == 18
output: def sum_integer(lst):
      return sum(lst)