Pattern matching with literals

During the coding of the Hood-Melville queue, I encountered an error message when I had put in a pattern matching an exact value of one of the parameters.
Simplifying the issue:
1) For this interpreter gives error:
```
data NoneNegativeInt = NNInt of Int

let subOne value =
match value with
| NNInt 0 => NNInt 0
| NNInt n => NNInt (n-1)
end
```
2. For this interpreter doesn't give error:
```
data NoneNegativeInt = Zero | NNInt of Int

let subOne value =
match value with
| Zero => Zero
| NNInt n => if n == 1 then Zero else NNInt (n - 1)
end
```
I think it would be great to be able to write matches like in the first point.