A Plug should be uniquely identifiable

[igormoreno]

Plugs are the endpoints of edges. They should be uniquely identifiable (i.e. contain unique ids). The code works now but the type doesn't reflect that uniqueness (currently Plug Int Int). The diagram could be created in an inconsistent state.

Currently, a Plug contains 2 Ints: one identifying the Node and another identifying the Hole within ​the Node. Pattern synonyms guarantee the second Int is appropriately set but not the first.

[igormoreno]

https://hackage.haskell.org/package/uniqueid-0.1.1/docs/Data-Unique-Id.html
"The main idea gows back to L. Augustsson, M. Rittri, and D. Synek and is described in their paper 'On generating unique names' (Journal of Functional Programming 4(1), 1994. pp. 117-123)."

State in Haskell
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.52.3656&rep=rep1&type=pdf

http://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Unique.html
(that one doesn't seem good because it creates an IO)