new_contents: from nose.tools import assert_equal
  from pomp.core.item import Item, Field
  
  
  def test_item():
      class TItem(Item):
          f1 = Field()
          f2 = Field()
          f3 = Field()
  
      i = TItem()
      assert_equal(['f1', 'f2', 'f3'], i.keys())
  
      i.f1 = 1
      i.f3 = 3
      assert_equal([1, None, 3], i.values())
  
subject: add test for core.item