code: def pipe(self, transformer):
  		
  		if self.next:
  			return
  
  		stream = Stream()
  		self.next = stream
  		stream.prev = self
  
  		self.transformer = transformer
  		transformer.stream = self
  		transformer.piped()
  
  		for file in self.files:
  			future = asyncio.ensure_future(self.transformer.transform(file))
  			future.add_done_callback(self.handle_transform)	
  
  		self.onpiped.set_result(None)
  		self.flush_if_ended()
  
  		return stream
docs: Pipe this stream to another.