`^` bug

Litex Code:
```litex
claim:
    forall x R:
        2 ^ 8 = 4 ^ x
        =>:
            x = 4
    prove:
        2 ^ 8 = (2 ^ 2) ^ 4 = 4 ^ 4 = 4 ^ x
        4 = x
```

Error:
```bash
4 = x is unknown :( line 8
```