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/usr/share/X11/xkb/compat/japan is in xkb-data 2.5-1ubuntu1.

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The actual contents of the file can be viewed below.

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// Japanese keyboards need Eisu and Kana shift and 
// lock keys, which are typically bound to the
// second shift level for some other modifier key.
// These interpretations disable the default
// interpretation (which would have these keys set
// the same modifier as the level one symbol).

default partial xkb_compatibility "japan"  {

    interpret.repeat= False;

    interpret Eisu_Shift+Lock {
	action= NoAction();
    };

    interpret Eisu_toggle+Lock {
	action= NoAction();
    };

    interpret Kana_Shift+Lock {
	action= NoAction();
    };

    interpret Kana_Lock+Lock {
	action= NoAction();
    };
};

// Some Japanese keyboards have an explict Kana Lock key & matching LED

partial xkb_compatibility "kana_lock"  {

    virtual_modifiers Kana_Lock;

    interpret Kana_Lock+AnyOfOrNone(all) {
        virtualModifier= Kana_Lock;
        useModMapMods=level1;
        action= LockGroup(group=+1);
    };

    indicator "Kana" {
        !allowExplicit;
	groups= All-Group1;
    };
};