I'd like to write my own layout where some character keys get remapped
as modifier keys. To keep things simple, I have made my layout a
variant of the "us". I have remapped the space bar as AltGr and it
works, Enter as Shift_L and it works too, but my attempts to remap
Control and Alt are ignored. Here are the relevant remappings:

partial alphanumeric_keys modifier_keys
xkb_symbols "my-layout" {

name[Group1]= "Italian - My Layout";

// Alphanumeric section
// Letters and controls (escape, delete, ecc) mappings omitted, they

key { [ Alt_L ] };
key { [ Alt_R ] };

// TAB works alone and shifted, not with Alt.
key { [ Tab, ISO_Left_Tab ] };

key { [ Control_R ] };

key { [ ISO_Level3_Shift, ISO_Prev_Group ] }; // OK

What I am missing?

I'm running Ubuntu 6.06.

Thanks for your help.