XKB: remapping Control and Shift does not work

Hi,

I've created my own keyboard layout where the Control is where Tab is,
and the Shift key is where 1 is. I've remapped Tab onto a letter key.
I've added the layout to /etc/X11/xkb/symbols. I'm using Ubuntu Linux
6.06 LTS.

Well, Control + Tab works, but Control + Shift + Tab does not. Here
is the layout:

partial alphanumeric_keys modifier_keys
xkb_symbols "my-layout" {

name[Group1]= "My layout";

// showing only relevant lines

key <AE01> { type[Group1]="ONE_LEVEL", [ Shift_L ] };
key <TAB> { type[Group1]="ONE_LEVEL",[ Control_L ] };
key <AB04> {type[Group1]="CTRL+ALT", [ Tab, ISO_Left_Tab ] };

modifier_map Shift { Shift_L, Shift_R };
modifier_map Lock { Caps_Lock, ISO_Lock };
modifier_map Control{ Control_L, Control_R };
modifier_map Mod1 { <ALT>, <LALT> };
modifier_map Mod1 { Alt_L, Meta_L };
};

Thanks in advance