code: def diff(var, key):
      '''calculate differences between values'''
      global last_diff
      ret = 0
      if not key in last_diff:
          last_diff[key] = var
          return 0
      ret = var - last_diff[key]
      last_diff[key] = var
      return ref diff(var, key):
      '''calculate differences between values'''
      global last_diff
      ret = 0
      if not key in last_diff:
          last_diff[key] = var
          return 0
      ret = var - last_diff[key]
      last_diff[key] = var
      return ret
docs: calculate differences between values