http://dswarmsikhttkg7jgsoyfiqpj3ighupfrvuz5ri3lu5q2dlqyrpgk7ad.onion/notes/lsg.html
“woof”, or sometimes “wiff”, “weff”, or “whiff” Lower-case letters are used to stand for formulas and upper-case letters are used for sets of formulas. (Huth, Michael and Ryan, Mark, 2004) If the set of all wffs of a formal language \(L\) is exactly the same as the set of all wffs of a formal language \(L'\), then \(L\) is the same formal language as \(L'\).