changing the transparency

I am trying to get a prim/texture to chance to see-through when touched
from 0% to 90%

I assime you use llSetTexture(); but have got stuck

default
{
state_entry()
{
llSay(0, "Hello, Avatar!";
}

touch_start(integer total_number)
{
llSay(0, "Touched.";
llSetTexture( Insert something to change the prim transparency );
}
}

Thanks for the help

This also turns it off and on when you touch it:

integer on_off = 1;

default {
        touch_start(integer n) {
            if(on_off)
                llSetAlpha(0.0, ALL_SIDES);
            else
                llSetAlpha(0.9, ALL_SIDES);
            on_off = !on_off;
        }
}

thanks perfect