Lisp has the unusual property that its programs take the form of parenthesized

lists. The natural syntax tree for a Lisp program is thus a tree of binary

cells (known in Lisp as cons cells), where the first child represents the first

element of the list and the second child represents the rest of the list. The

syntax tree for (cdr (a b c)) appears in Figure 4.16. (The notation L is

syntactic sugar for (quote L).)

Extend the CFG of Exercise 2.18 to create an attribute grammar that will

build such trees. When a parse tree has been fully decorated, the root should

have an attribute v that refers to the syntax tree. You may assume that each

atom has a synthesized attribute v that refers to a syntax tree node that holds

information from the scanner. In your semantic functions, you may assume

the availability of a cons function that takes two references as arguments

and returns a reference to a new cons cell containing those references.

2.18)

P E $$

E atom

E

( E Es )

Es E Es