(This is the thirteenth lecture in Stevo Todorcevic’s Forcing class, held in the fall of 2012. You can find the twelfth lecture here. Quotes by Stevo are in dark blue; some are deep, some are funny, some are paraphrased so use your judgement. As always I appreciate any type of feedback, including reporting typos, in the comments below.)

**Summary**

- We restate the strong lemma.
- A handful of spaces are presented to illustrate the following theorem.
- A theorem about Baire-Class-1 functions is stated. (Which is proved by forcing!)
- The proof strategy is discussed.