Finding the index of the maximum value in an associative array

[Lyle via Digitalmars-d-learn]

Hi,

I have an associative array of type int[ulong] and I'm trying to 
get the index of the maximum value, like this:

int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];

writeln(aa.maxIndex); // should print 2

Can anyone help me out?

Many thanks,
Lyle