Extendable Enum like Type?

[Antoon Pardon]

Something seems to have gone wrong with the formatting of my latest contribution,
so let me try again.

I am experimenting with writing an Earley Parser. Now I would like to
have the non-terminals from the grammer I am reading in, be represented
bye an enum like type. So that if the grammer contains the following
production: Term -> Term '+' Factor I can reprensent the right hand side
with a list that gets printed something like the following:
[<Non_Terminal.Term:1>, '+', <Non_Terminal.Factor:2>] I am a bit at a
loss right now on how to start. Can someone point me in the right
direction? -- Antoon Pardon