How to make a F# discriminated union out of another type's union cases? -

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imagine discriminated union:

type direction =      | north     | south     | east     | west

now imagine want type accepts tuples of (north, south) or (east, west). perhaps describe train routes run north south, or east west. (north, east) , (south, west) should forbidden, perhaps because trains don't run that.

this doesn't work:

type trainlines =      | north, south     | east, west

even though doesn't work, perhaps can see i'm trying do.

this works, doesn't restrict possibilites (north, south) , (east, west):

type trainlines = direction * direction

any guidance welcomed.

this isn't asked for, think it's 

type trainlines =     | northsouth     | eastwest

would good. if needed add e.g.

    member this.directions =            match            | northsouth -> [north; south]            | eastwest -> [east; west]

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