F# probabilities: opposed 3d6 rolls

A handy little F# script for computing odds for Dungeon Fantasy/GURPS, such as the odds of a Slow Stone affecting a dragon:

let distribution3d6 =
    [for x in 1..6 do
        for y in 1..6 do
            for z in 1..6 do
                (x+y+z), (x,y,z)]
let probabilityOfSuccess attackerSkill defenderSkill =
    let mutable n = 0
    let mutable success = 0
    for x,_ in distribution3d6 do
        for y,_ in distribution3d6 do
            n <- n + 1
            let successMargin =
                match attackerSkill - x with
                | n when n >= 0 -> Some n
                | _ -> None
            match defenderSkill - y, successMargin with
            | defenderMargin, Some attackerMargin when attackerMargin > defenderMargin ->
                success <- success + 1
            | _ -> ()
    {| attackerSkill = attackerSkill; defenderSkill = defenderSkill; n = n; success = success; successRatio = (float success/float n) |}
probabilityOfSuccess 15 15

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