Because symbolic evaluation requires that each variable or expression has a value, it is necessary to add to REDUCE the concept of a quoted expression by analogy with the Lisp QUOTE function. This is provided by the single quote mark ’. For example,
|’a||represents the Lisp S-expression||(quote a)|
|’(a b c)||represents the Lisp S-expression||(quote (a b c))|
Note, however, that strings are constants and therefore evaluate to themselves in symbolic mode. Thus, to print the string ~A String~, one would write
Within a quoted expression, identifier syntax rules are those of REDUCE. Thus (A !. B) is the list consisting of the three elements A, ., and B, whereas (A . B) is the dotted pair of A and B.