input: Write a function to find the sum of a given list of numbers. If the list is empty, it should return 0.
  
  assert sum_list([1, 2, 3, 4])==10
  assert sum_list([5, 10, 15])==30
  assert sum_list([-1, 0, 1])==0
  assert sum_list([10])==10
  assert sum_list([])==0
output: def sum_list(numbers):
      if not numbers:
          return 0
      return sum(numbers)