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([2, 4, 6, 8])==20
  assert sum_sequence([10])==10
  assert sum_sequence([])==0
  assert sum_sequence([-1, -2, -3])==-6
output: def sum_sequence(seq):
      return sum(seq)