code: def pop(self):
          
  
          if self.reading is None:
              raise StreamEmptyError("Pop called on virtual stream walker without any data", selector=self.selector)
  
          reading = self.reading
  
          # If we're not a constant stream, we just exhausted ourselves
          if self.selector.match_type != DataStream.ConstantType:
              self.reading = None
  
          return reading
docs: Pop a reading off of this virtual stream and return it.