Immutability, problems with debugging

[Nathan M. Swan]

I'm working on a novice solver of the travelling salesman 
problem, and when I decided to go concurrent I had to make 
everything immutable. Now this happens:

        /usr/share/dmd/src/phobos/std/format.d(1782): Error: 
function object.Object.toString () is not callable using argument 
types () immutable

Unfortunately, I don't know how format is called, so I can't 
investigate closer. Does anyone notice anything glaringly obvious?

https://github.com/carlor/tsp.d/blob/master/tsp.d

This brings up an issue that's happened before; there's an error 
in the semantics of a (usually generic) function call, and I 
don't know where the function is called. It would be nice if the 
compiler could do something like "mentioned on file1.d(33), 
mentioned on file2.d(107)".

Thanks, NMS