memcache.go
func (c *Client) Add(item *Item) error {
	return c.populateOne(cmdAdd, item, 0)
}
func (c *Client) Add(item *Item) error {
	return c.populateOne(cmdAdd, item, 0)
}
// Add writes the given item, if no value already exists for its
// key. ErrNotStored is returned if that condition is not met.