code: func SandboxKeyWalker(out *Sandbox, key string) SandboxWalker {
  	return func(sb Sandbox) bool {
  		if sb.Key() == key {
  			*out = sb
  			return true
  		}
  		return false
  	}
  }
docs: // SandboxKeyWalker returns a Sandbox Walker function which looks for an existing Sandbox with the passed key