code: func First(n int) Filter {
  	return FilterFunc(func(arg Arg) error {
  		seen := 0
  		for s := range arg.In {
  			if seen >= n {
  				break
  			}
  			arg.Out <- s
  			seen++
  		}
  		return nil
  	})
  }
docs: // First yields the first n items that it receives.