input: Write a function to calculate the sum of a sequence of integers.
  
  assert sum_sequence([1, 2, 3, 4, 5])==15
  assert sum_sequence([1, 2, 3, 4, 5, 6])==21
  assert sum_sequence([])==0
  assert sum_sequence([10, 20, 30])==60
  assert sum_sequence([-1, 0, 1])==0
output: def sum_sequence(seq):
      return sum(seq)