code: func filterTargets(
  	wcs []io.WriteCloser,
  	line []byte,
  	filter func(io.WriteCloser, []byte) bool,
  ) []io.WriteCloser {
  	var filteredTargets []io.WriteCloser
  
  	for _, c := range wcs {
  		if !filter(c, line) {
  			filteredTargets = append(filteredTargets, c)
  		}
  	}
  	return filteredTargets
  }
docs: // filterTargets passes line to each writer in wcs,
  // filtering out single writers if filter returns true.