Calculating prime (Optimus?)

Instead of needing to keep track of whether it's a prime with `prime`, it might be simpler to label the outer loop and use a `continue LABEL` statement. Then it can simply be:

```java
PRIMELOOP:
for ( int i = start; i <= end; i++ ) {
  //...
    if ( i % j == 0 )
      continue PRIMELOOP;
  }
  primes.add( i );
}
```

Unless you feel like labeled `continue`s are too close to GOTO's for comfort.