LISP I Programmer's Manual - Software Preservation Group

Subexpressions can be similarly abbreviated. Some examplesof these abbreviations are((AB,c),D) for ((AB- (c*NIL))* (DONIL))((A,B),c,D*E) for ((A* (B-NIL)) (c- (DOE)))Since we regard the expressions with commas as abbreviationsfor those not involving commas, we shall refer to them all as S-expressions.b. Functions of S-expressions and the Expressions That RepresentThemWe now define a class of functions of S-expressions. Theexpressions representing these functions are written in a conventionalfunctional notation. However, in order to clearlydistinguish the expressions representing functions from S-expressions,we shall use sequences of lower-case letters forfunction names and variables ranging over the set of S-expressions.We also use brackets and semicolons, instead of parenthesesand commas, for denoting the application of functionsto their arguments. Thus we writecar[xlcar[cons[ (A-B);x] 1In these M-expressions (meta-expressions) any S-expressions thatoccur stand for themselves.c. The Elementary S-functions and PredicatesWe introduce the following functions and predicates:1. atomatom[x] has the value of T or F, accordingly as x is anatomic symbol or not. Thus