std.algorithm range violation

[Wanderer via Digitalmars-d-learn]

Aha, so you want to maintain "spam word" -> "set of senders" 
relationship, so it's actually "map of sets" and your declaration 
is correct. You only need to sort map's entries (key and value 
pairs together).

If D supports that, it should be something like 
providor_symbol_map.sort!((x,y)=>{x.value.length=>y.value.length}), 
but I'm not sure it would work...

But I'm just learning D (and very much like it so far!), 
hopefully someone more skilled will reply as well. :-)