graph that is represented as associative array
A graph series composed of keys (sorted .index) and arrays of indeces (.data) Complexity: O(log(V) (V + E))
auto gs = [ "b" : ["a"], "a" : ["b", "c"], "c" : ["b"], ].graphSeries; assert (gs.index == ["a", "b", "c"]); // sorted assert (gs.data == [ [1, 2], // a [0], // b [1], // c ]);