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## 14.1.1 Conses as Trees

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.

    caaaar  caddar  cdar       nsubst
    caaadr  cadddr  cddaar     nsubst-if
    caaar   caddr   cddadr     nsubst-if-not
    caadar  cadr    cddar      nthcdr
    caaddr  cdaaar  cdddar     sublis
    caadr   cdaadr  cddddr     subst
    caar    cdaar   cdddr      subst-if
    cadaar  cdadar  cddr       subst-if-not
    cadadr  cdaddr  copy-tree  tree-equal
    cadar   cdadr   nsublis

Figure 14-2. Some defined names relating to trees.

## 14.1.1.1 General Restrictions on Parameters that must be Trees

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