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You can find a list of options in `man xkeyboard-config`.

Under the section "key to choose the 3rd level" is the one you are after:

    lv3:ralt_switch_multikey      Right Alt; Shift+Right Alt as Compose

To enable it, use

    setxkbmap -option lv3:ralt_switch_multikey

You should also pass other options you already use to `-layout` or they will be reverted.

To make it permanent, add it to the comma separated XKBOPTIONS variable
in `/etc/default/keyboard`, e.g.:

    XKBMODEL="pc105"
    XKBLAYOUT="en"
    XKBVARIANT=""
    XKBOPTIONS="lv3:ralt_switch_multikey"
    BACKSPACE="guess"

-----

That option is defined in /usr/share/X11/xkb/symbols/level3 as

    // The right Alt key (while pressed) chooses the third shift level,
    // and Compose is mapped to its second level.
    partial modifier_keys
    xkb_symbols "ralt_switch_multikey" {
      key <RALT> {
        type[Group1]="TWO_LEVEL",
        symbols[Group1] = [ ISO_Level3_Shift, Multi_key ]
      };
      include "level3(modifier_mapping)"
    };

In general, if the desired combination weren't available, you could create
your own map. E.g., to make the menu key* have that behavior,

    ==> ~/.config/xkb/symbols/mymap <==

    partial alphanumeric_keys modifier_keys
    xkb_symbols "acer" {
        key <MENU> {
            type="TWO_LEVEL",
            symbols[Group1] = [ ISO_Level3_Shift, Multi_key ]
        };
    };

Then create this one from `setxkbmap -print` and append your custom option
to `xkb_symbols`:

    ==> ~/.config/xkb/main <==

    xkb_keymap {
        xkb_keycodes{include "evdev+aliases(qwerty)"};
        xkb_types   {include "complete"};
        xkb_compat  {include "complete"};
        xkb_symbols {include "pc+en+inet(evdev)+mymap(mymap)"};
        xkb_geometry{include "pc(pc105)"};
    };

Finally, run

    xkbcomp -I"$HOME/.config/xkb" "$HOME/.config/xkb/main" "$DISPLAY"

at every X session start to put load that custom key map.

*`xkbcomp "$DISPLAY" -` outputs your full key map; useful to find a key name.