Setting a "compose" key: § MATEFrom System -> Preferences -> Hardware -> Keyboard, Layouts -> Options, put Compose where you want it. I've got it on right-Alt.
§ GNOME Settings > Keyboard > Compose Key Activate and select key to use. Full list of predefined compose keys can be found here: /usr/share/X11/locale/en_US.UTF-8/Compose ~~R
