D graph library

[w0rp]

On the subject of possibly making addNode nothrow, I think it may 
be difficult or impossible at current to make the Graph functions 
pure, which I think is rather unfortunate. At my last try in D, I 
backed my graphs with Weight[Node][Node] exactly as mentioned 
before, but this ruined the purity. This was all because the 
properties for associative arrays are not pure, which is one of 
my top change requests for D. I really would like to see a graph 
type that is usable inside pure functions.