input: Write a function to calculate the sum of a sequence of integers. If the input sequence is empty, return 0.
  assert sum_sequence([1, 2, 3, 4, 5]) == 15
  assert sum_sequence([-1, -2, -3, -4, -5]) == -15
  assert sum_sequence([1]) == 1
  assert sum_sequence([]) == 0
output: def sum_sequence(sequence):
      return sum(sequence)