input: Write a python function to print a sequence of numbers.
  assert print_sequence() == "1\n12\n123\n1234\n12345\n123456\n"
  assert print_sequence() == "1\n12\n123\n1234\n12345\n123456\n"
  assert print_sequence() == "1\n12\n123\n1234\n12345\n123456\n"
output: def print_sequence():
      result = ""
      for i in range(1, 7):
          for j in range(1, i + 1):
              result += str(j)
          result += "\n"
      return result