new_contents: import unittest
  import datetime
  import hiro
  from sifr.span import Minute, Year, Month, Day, Hour
  
  
  class SpanTests(unittest.TestCase):
      def test_minute(self):
          with hiro.Timeline().freeze(datetime.datetime(2012, 12, 12)):
              now = datetime.datetime.now()
              span = Minute(now, ["single"])
              self.assertEqual(span.key, "single:2012-12-12_00:00")
  
      def test_hour(self):
          with hiro.Timeline().freeze(datetime.datetime(2012, 12, 12)):
              now = datetime.datetime.now()
              span = Hour(now, ["single"])
              self.assertEqual(span.key, "single:2012-12-12_00")
  
      def test_day(self):
          with hiro.Timeline().freeze(datetime.datetime(2012, 12, 12)):
              now = datetime.datetime.now()
              span = Day(now, ["single"])
              self.assertEqual(span.key, "single:2012-12-12")
  
      def test_month(self):
          with hiro.Timeline().freeze(datetime.datetime(2012, 12, 12)):
              now = datetime.datetime.now()
              span = Month(now, ["single"])
              self.assertEqual(span.key, "single:2012-12")
  
      def test_year(self):
          with hiro.Timeline().freeze(datetime.datetime(2012, 12, 12)):
              now = datetime.datetime.now()
              span = Year(now, ["single"])
              self.assertEqual(span.key, "single:2012")
  
subject: Add basic tests for spans