This is amazing! I don't think I'll use it for help (up-arrow, ctrl+w is fewer keystrokes), but I can think of many other situations where I want to substitute

This is amazing! I don't think I'll use it for help (up-arrow, ctrl+w is fewer keystrokes), but I can think of many other situations where I want to substitute one word of a very long command and didn't realize it could be this easy