I am trying to test the equality of the elements in a tuple of type char using pattern matching as follows: 

```
let swap (x,y) =
match fst(x,y) with
| snd(x,y) -> (x,y)
| _ -> (y,x);;

```

and receive the following error: `stdin(11,8): error FS0039: The pattern discriminator 'snd' is not defined`

Please note, I have already discovered a better way to solve the actual problem. I am just curious why this approach doesn't work.