Did you get that one?

Position.fs

// Value objects à la F#
type Position =
    struct
        val Value : int64 
        new (value) = { Value = value } then 
            if value < 0L then invalidArg "value" "The value must be greater than or equal to 0."
        member this.Next =
            if this.Value = Int64.MaxValue then
                invalidOp (sprintf "There's no next value beyond %d" Int64.MaxValue)
            new Position(this.Value + 1L)
        static member (+) (left: Position, right: Position) =
            new Position(left.Value + right.Value)
        static member (+) (left: Position, right: int32) =
            new Position(left.Value + int64(right))
        static member (+) (left: Position, right: int64) =
            new Position(left.Value + right)
        static member Zero = new Position(0L)
        static member One = new Position(1L)
    end

//Usage
let position = Position.Zero
let range = seq { position .. position + 5 } // nice

/*
** I started out without the (+) operator, compiler started complaining at "seq { position"
** that I was missing that operator overload. Once fixed it started complaining at "seq { position"
** again, now that the get_One operator was missing. Ruben pointed me in the right direction,
** which seems obvious in hindsight. I like the terse F# syntax I ended up with.
*/