std.algorithm range violation

[Wanderer via Digitalmars-d-learn]

On Wednesday, 28 May 2014 at 10:10:41 UTC, maarten van damme via
Digitalmars-d-learn wrote:
> an anyone explain me what I'm doing wrong here :
>
> [code]
> dstring[][dstring] providor_symbol_map;
> ...
>
> writeln(providor_symbol_map.keys.sort!((x,y)=>providor_symbol_map[x].length>=providor_symbol_map[y].length));
> [/code]
>
> output:
> core.exception.RangeError at std.algorithm(9429): Range violation

"dstring[][dstring]" declaration looks a bit obscure to me...
what do you intent with it, "array of maps" or "map of arrays"?

Also, it looks unnatural that inside sorting lambda, you refer
outside from it (back to providor_symbol_map variable). Ideally,
you should only use variables x and y. Does D support sorting by
map entries instead of by keys only or by values only?