Ord for partially ordered sets

[Henning Thielemann]

On Fri, 24 Apr 2015, Ivan Lazar Miljenovic wrote:

> Specifically, I have a pull request for fgl [1] to add Ord instances for 
> the graph types (based upon the Ord instances for Data.Map and 
> Data.IntMap, which I believe are themselves partially ordered), and I'm 
> torn as to the soundness of adding these instances.

In an application we needed to do some combinatorics of graphs and thus 
needed Set Graph.

Nonetheless, I think that graph0 < graph1 should be a type error. We can 
still have a set of Graphs using a newtype.