I have such a function:

```
let RotateFace (face: int, turns: int) =
    match face with
        | upperface -> 
            TiltCube(2)
            TwistCube(turns)
            TiltCube(2)
        | leftface -> 
            TiltCube(3)
            TwistCube(turns)
            TiltCube(1)
        | frontface -> 
            TurnCube(1)
            TiltCube(3)
            TwistCube(turns)
            TiltCube(1)
            TurnCube(3)
        | rightface -> 
            TiltCube(1)
            TwistCube(turns)
            TiltCube(3)
        | backface -> 
            TiltCube(3)
            TwistCube(turns)
            TiltCube(1)
        | downface -> 
            TurnCube(3)
            TiltCube(3)
            TwistCube(turns)
            TiltCube(1)
            TurnCube(1)
        | _ -> ()

```

I have a "this rule will never be matched" problem on my cases "'upperface', 'leftface', 'frontface', 'rightface', 'backface', 'downface', and '\_'.".

I've looked this link;

[f# match expression - "rule will never be matched"](https://stackoverflow.com/questions/18984535/f-match-expression-rule-will-never-be-matched)

but to be honest I didn't understand what should I do with my case.