http://dswarmsikhttkg7jgsoyfiqpj3ighupfrvuz5ri3lu5q2dlqyrpgk7ad.onion/notes/lsg.html
Jaśkowski style proof 13.23. transitive closure 14. Natural Deduction (Huth, Michael and Ryan, Mark, 2004) 14.1. Rules for natural deduction (Transformation rules) 14.1.1. Rules of Inference 14.1.2. Rules of replacement 1. The Guide 2. Informal Set Theory 2.1.