Data.Graph transitive closure

[Johannes Waldmann]

> I've actually wondered why Data.Graph existed since it is obviously not
> written for serious/heavy usage. 

But it was!

As I understand, it was originally part of GHC
(ghc-0.29/ghc/compiler/utils/Digraph.lhs)
for computing SCCs (in the graph of dependencies of declarations).

So we can assume that it does this well.

And I guess it's used for the very same purpose in Cabal and Agda.

- J.