string problem

From: x-bit (user_at_example.net)
Date: 04/23/04 

Date: Fri, 23 Apr 2004 14:51:16 +0200

variable foo "456"
variable x ""
set a {
        "A: 123"
        "B: $foo"
}
set x [lindex $a [rand [llength $a]]]

-----------
x ->

A: 123
B: $foo (should be B: 456, but i don't get it)