Internally in the metacircular evaluator (after parsing), every Pico program that meets the concrete grammar will be represented by a parse-tree. The rules that tell us which trees are valid parse trees and which trees are not, are called the abstract syntax used by this particular Pico implementation. The following lines are 'rules' that tell us how trees are to be composed in order to be valid abstract syntax trees. The actual composition of the trees is accomplished by the reader. The reader therefore uses the 'tree ADT' defined by the actual constructors for the trees (these are defined in Grammar.pco). Hence, the code in the file Grammar.pco is merely a collection of constructors and accessors for the abstract rules given in this file.