input: Write a function to find the sum of a list of numbers. The function should take a list of numbers as input and return the sum of its elements. If the input list is empty, the function should return 0.
  
  assert sum_list([1, 2, 3])==6
  assert sum_list([5, 10, 15, 20])==50
  assert sum_list([])==0
  assert sum_list([-1, -2, -3])==-6
output: def sum_list(numbers):
      if not numbers:
          return 0
      return sum(numbers)