Although I think the API with the dictionary may be cleaner, I still do not like it completely.

1) One of the reasons is that implementing C3 directly with the topological sort is not straightforward as the different nodes that are at the same level are unordered and therefore any permutation of all these is a valid topological sort, while C3 comes from a stable sort. The version with pairs is stable on the other hand.

2) Topological sorting usually is well-defined on totally connected graphs, so I do not know what exactly it means to topologically sort two disjoint graphs. This was one of the main drawbacks of the tuple-based approach, but I think it may be a good property.