# Fall 2019 edition of forall x:YYC is out

The Fall 2019 edition of forall x: Calgary is officially out. It’s available for download at forallx.openlogicproject.org and can be purchased in print at Amazon (US | CA | UK).

• New cover design by Mark Lyall.
• Rewritten/revised chapters 1–3, avoiding modality when defining validity and instead talk about “cases” (the way JC Beall and Shay Logan do in their book, Logic: The Basics).
• Changed the syntax of FOL: formulas are now F(x, y) instead of Fxy. This can be customized in forallxyyc-local.sty, though.
• Changed some terminology: “logical consistency” is now “satisfiability” (and cognates), “logical truths” are now “validities”.
• Sentence letters now allowed in FOL formulas.
• Reiteration is now a basic rule, so that proofs can be in normal form.
• Chapter 28 now defines satisfaction of a formula by an object, so that truth conditions for quantifiers can be stated more cleanly.
• New chapters 16, 33 on strategies for constructing proofs.
• Added three chapters on modal logic (natural deduction proof system and Kripke semantics). They are based on a Modal Logic Primer by Rob Trueman (University of York), which he was generous enough to share. I’ve changed his proof system to use Fitch’s modal rules. It covers the basics of K, T, S4, and S5.
• Added a chapter on soundness for TFL proofs, from Tim Button’s Metatheory.
• Corrected various errors and added various clarifications.