code: func OptFieldTypeMutex(cacheType CacheType, cacheSize int) FieldOption {
  	return func(options *FieldOptions) {
  		options.fieldType = FieldTypeMutex
  		options.cacheType = cacheType
  		options.cacheSize = cacheSize
  	}
  }
docs: // OptFieldTypeMutex adds a mutex field.