UNTIL will break at first chance.

If the UNTIL condition ever becomes true the loop will break even if not completely executed.

```
 SET a TO 5.
 UNTIL a<0.5 {
   PRINT a.
   SET b TO a - 1.
   SET a TO 0.
   SET a TO b - 1.
 }.
 PRINT "Done: " + a.
```

```
5
Done: 0
```

Can't figure out a good way to find out where in the block we're at.
