5. PRIMITIVE SYMBOLS, AND FORMULAS. We turn now to the development of a formal system, which we shall call the calculus of A-conversion, and which shall have as a possible interpretation or application the system of ideas about functions described in Chapter I.
The primitive symbols of this calculus are three symbols,
۸, (, ),
which we shall call improper symbols, and an infinite list of symbols,
a, b, c, ... , x, y, z, ā, Ƃ, ... , ẑ, ǟ, ... ,
which we shall call variables. The order in which the variables appear in this originally given infinite...