FQN error in `to-java.xsl`

This is my code:

```
[args] > program
  io.stdout > @
    txt.sprintf *1
      "Hello, %s\n"
      args.at 0
```

It doesn't compile with this message:

```
FQN must start with either with Φ or ξ, but txt found
```

The message is pretty weird. Why did we not catch the problem earlier and produced a broken XMIR? Also the message is grammatically incorrect.