For instance, the following box-and-arrow diagram shows the cons cells that make up the list of lists: ((1 2) (3 4) (5 6)). The list structure includes only the three cons cells inside the dashed box while the tree structure includes all the cons cells.

**COPY-TREE**, on the other hand, makes a new cons cell for each of the cons cells in the diagram and links them together in the same structure, as shown in this diagram:

Where a cons cell in the original referenced an atomic value, the corresponding cons cell in the copy will reference the same value. Thus, the only objects referenced in common by the original tree and the copy produced by **COPY-TREE** are the numbers 1-6, and the symbol **NIL**.

Another function that walks both the **CAR**s and the **CDR**s of a tree of cons cells is **TREE-EQUAL**, which compares two trees, considering them equal if the tree structure is the same shape and if the leaves are **EQL** (or if they satisfy the test supplied with the :test keyword argument).

**SUBST-IF** is analogous to **SUBSTITUTE-IF**. Instead of an old item, it takes a one-argument function—the function is called with each atomic value in the tree, and whenever it returns true, the position in the new tree is filled with the new value. **SUBST-IF-NOT** is the same except the values where the test returns **NIL** are replaced. **NSUBST**, **NSUBST-IF**, and **NSUBST-IF-NOT** are the recycling versions of the **SUBST** functions. As with most other recycling functions, you should use these functions only as drop-in replacements for their nondestructive counterparts in situations where you know there’s no danger of modifying a shared structure. In particular, you must continue to save the return value of these functions since you have no guarantee that the result will be to the original tree.2