docs: def readReference(self):
          """
          Reads a reference from the data stream.
  
          @raise pyamf.ReferenceError: Unknown reference.
          """
          idx = self.stream.read_ushort()
          o = self.context.getObject(idx)
  
          if o is None:
              raise pyamf.ReferenceError('Unknown reference %d' % (idx,))
  
          return o