This chapter describes procedures available for manipulating trees. The term tree is used in a technical sense to refer to non-circular list structure considered as tree data-structures, as contrasted with lists, which use chained pairs to represent a one-dimensional sequence of objects. Thus one might say either that (A (B C) D) is a list of three elements, or that it is a tree with four (non-null) leaves.
Trees are used to represent programs. The tree-manipulation procedures and special forms are designed with this in mind.