... formulas of LP in any one of theschemas A1 , A2 , or A3 gives an axiom of LP.For example, (A 1→ (A 4→ A 1)) is an axiom, being an instance ofaxiom schema A1 , but (A 9 ( A 0)) is not an axiom ... structures and, in each case, an appropriate set of axioms in yourlanguage: PREFACE viiparticular, a number of the key papers in the development of modern mathematical logic can be found in [9]and[6].Others ... formalize almost all of mathematics in a single first-orderlanguage, like that of set theory or category theory. However, trying to actually domost mathematics in such a language is so hard as...