input: 
  Write a function to calculate the sum of elements in a given list of numbers. If the list is empty, return 0.
  
  assert sum_list([1, 2, 3, 4])==10
  assert sum_list([-1, 1, -1, 1])==0
  assert sum_list([5, 5, 5, 5])==20
  assert sum_list([])==0
  assert sum_list([10])==10
  
output:     
  def sum_list(numbers: List[int]) -> int:
      if not numbers:
          return 0
      return sum(numbers)
  