break, and the continue You Are Not Implementing

The point

break is the first construct whose legality depends on context rather than on its own text. That makes it a small, concrete example of why phase 3 exists.

Do this

1. Predict the target of each break below, then implement the label stack and check.

   while (a) {
       while (b) {
           break;        <- which loop does this leave?
       }
       break;            <- and this one?
   }

2. Implement the semantic check. A break outside any loop must be an error with a line number. Use a depth counter incremented around loop bodies only — and explain in one sentence why the condition is excluded.
3. Test the error with a top-level break; and confirm the message names the line.
4. Argue from theory. Write two sentences explaining why no context-free grammar could have caught this, referring to what the production for break_stmt can and cannot see.
5. Design labeled break. Java has break outer;. What would that require of your label stack? Two sentences.