http://dswarmsikhttkg7jgsoyfiqpj3ighupfrvuz5ri3lu5q2dlqyrpgk7ad.onion/notes/lsg.html
Recommendations on informal basic set theory 2.4. Virtual classes, real sets 3. First-order logic 3.1. Propositional logic 3.1.1. Syntax propositional ‘atomic’ wffs: \(\mathsf{P, Q, R}\), … logical operators: \(\land, \lor, \to\) and \(\neg\) always-false absurdity constant: \(\bot\) 3.1.2.