Global variables read at compile time?

[Stefan]

Hi there, I'm having trouble getting the following code to 
compile:

import std.stdio;

string a = "a";
string b = a;

void main()
{
     writeln(b);
}

DMD spits out the error "test.d(4): Error: variable a cannot be 
read at compile time". Is there any way to tell the compiler I 
want b evaluated at runtime, or am I missing something obvious 
here?