{\displaystyle {\mathcal {A}}} we mean a nonempty set of symbols. Rautenberg, Wolfgang. A Concise Introduction to Mathematical Logic (PDF) Third. Springer
Publications）, 2002 [1967], ISBN 978-0-486-42533-7, MR 1950307 Rautenberg, Wolfgang, A Concise Introduction to Mathematical Logic 3rd, New York: Springer