sorting failed error

[maarten van damme]

For fun I started implementing a program that uses genetics
algorithm's to solve the travelling salesman problem. I want to sort
an array of chromosome structures according to their fitness. I now
get the error:

core.exception.AssertError at C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.
d(6714): Failed to sort range of type chromosome[]. Actual result is: [chromosom
e([city(20, 0), city(25, 25), city(10, 65), city(50, 50), city(75, 30), city(0,
10)]), chromosome([city(10, 65), city(50, 50), city(25, 25), city(75, 30), city(
...

I have no idea what is wrong with my code and the error is not very informative.
Does anyone have any idea as to what the problem could be?

(full code is at
https://dl.dropbox.com/u/15024434/travelling_salesman.d , If it's not
something obvious then I'm going to try to reduce it to something as
small as possible)

Maarten