A tree is a binary recursive data structure made up of conses and atoms: the conses are themselves also trees (sometimes called “subtrees” or “branches”), and the atoms are terminal nodes (sometimes called leaves). Typically, the leaves represent data while the branches establish some relationship among that data.
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Figure 14.2: Some defined names relating to trees.
Except as explicitly stated otherwise, for any standardized function that takes a parameter that is required to be a tree, the consequences are undefined if that tree is circular.