You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: with_pool.go
    path of file: ./repos/athens/pkg/stash
    
    the code of the file until where you have to start completion: package stash

import (
	"context"

	"github.com/gomods/athens/pkg/errors"
	"github.com/gomods/athens/pkg/observ"
)

type withpool struct {
	stasher Stasher

	// see download/addons/with_pool
	// for design docs on about this channel.
	jobCh chan func()
}

// WithPool returns a stasher that runs a stash operation
// {numWorkers} at a time.
func WithPool(numWorkers int) Wrapper {
	return 
    
func (s *withpool) Stash(ctx context.Context, mod, ver string) (string, error) {
	const op errors.Op = "stash.Pool"
	ctx, span := observ.StartSpan(ctx, op.String())
	defer span.End()
	var err error
	var newVer string
	done := make(chan struct{}, 1)
	s.jobCh <- func() {
		newVer, err = s.stasher.Stash(ctx, mod, ver)
		close(done)
	}
	<-done
	if err != nil {
		return "", errors.E(op, err)
	}

	return newVer, nil
}