See http://hackage.haskell.org/trac/ghc/ticket/1266 for the patch. The
basic idea is that insMapNode should survive a duplicate node, and
insMapEdge should survive (and insert) an edge whose endpoints weren't
yet in the graph. This was impossible for the current type of
insMapEdge, so I added a safeInsMapEdge instead.

There's also a fairly small test suite in here, which people are
welcome to suggest improvements to.

I suggest a discussion period of two weeks, ending April 17, unless
discussion is still active by then, in which case a week after it dies
down.

Thanks,
Jeffrey Yasskin