grep "grp:." /usr/share/X11/xkb/rules/base.lst

  grp:switch           Right Alt (while pressed)
  grp:lswitch          Left Alt (while pressed)
  grp:lwin_switch      Left Win (while pressed)
  grp:rwin_switch      Left Win (while pressed)
  grp:win_switch       Any Win (while pressed)
  grp:menu_switch      Menu (while pressed), Shift+Menu for Menu
  grp:caps_switch      Caps Lock (while pressed), Alt+Caps Lock for the original Caps Lock action
  grp:rctrl_switch     Right Ctrl (while pressed)
  grp:toggle           Right Alt
  grp:lalt_toggle      Left Alt
  grp:caps_toggle      Caps Lock
  grp:shift_caps_toggle Shift+Caps Lock
  grp:shift_caps_switch Caps Lock to first layout; Shift+Caps Lock to last layout
  grp:win_menu_switch  Left Win to first layout; Right Win/Menu to last layout
  grp:lctrl_rctrl_switch Left Ctrl to first layout; Right Ctrl to last layout
  grp:alt_caps_toggle  Alt+Caps Lock
  grp:shifts_toggle    Both Shift together
  grp:alts_toggle      Both Alt together
  grp:ctrls_toggle     Both Ctrl together
  grp:ctrl_shift_toggle Ctrl+Shift
  grp:lctrl_lshift_toggle Left Ctrl+Left Shift
  grp:rctrl_rshift_toggle Right Ctrl+Right Shift
  grp:ctrl_alt_toggle  Alt+Ctrl
  grp:alt_shift_toggle Alt+Shift
  grp:lalt_lshift_toggle Left Alt+Left Shift
  grp:alt_space_toggle Alt+Space
  grp:menu_toggle      Menu
  grp:lwin_toggle      Left Win
  grp:win_space_toggle Win+Space
  grp:rwin_toggle      Right Win
  grp:lshift_toggle    Left Shift
  grp:rshift_toggle    Right Shift
  grp:lctrl_toggle     Left Ctrl
  grp:rctrl_toggle     Right Ctrl
  grp:sclk_toggle      Scroll Lock
  grp:lctrl_lwin_rctrl_menu Left Ctrl+Left Win to first layout; Right Ctrl+Menu to second layout
  grp:lctrl_lwin_toggle Left Ctrl+Left Win

// Control_L+Win_L toggles groups.
partial modifier_keys
xkb_symbols "lctrl_lwin_toggle" {
    key <LWIN> {
        type[ Group1 ]    = "PC_CONTROL_LEVEL2",
        symbols[ Group1 ] = [ Super_L,    ISO_Next_Group ]
    };
};

// Win_L+Control_L toggles groupd.
partial modifier_keys
xkb_symbols "lwin_lctrl_toggle" {
    virtual_modifiers AltGr;
    key <LWIN> {
        virtualMods= AltGr,
        type[ Group1 ]    = "PC_CONTROL_LEVEL2",
        symbols[ Group1 ] = [ Super_L,    ISO_Next_Group ]
    };
};