#### 9.2.3 Containers

Two common container data structures can be represented directly with tree nodes. A `TREE_LIST` is a singly linked list containing two trees per node. These are the `TREE_PURPOSE` and `TREE_VALUE` of each node. (Often, the `TREE_PURPOSE` contains some kind of tag, or additional information, while the `TREE_VALUE` contains the majority of the payload. In other cases, the `TREE_PURPOSE` is simply `NULL_TREE`, while in still others both the `TREE_PURPOSE` and `TREE_VALUE` are of equal stature.) Given one `TREE_LIST` node, the next node is found by following the `TREE_CHAIN`. If the `TREE_CHAIN` is `NULL_TREE`, then you have reached the end of the list.

A `TREE_VEC` is a simple vector. The `TREE_VEC_LENGTH` is an integer (not a tree) giving the number of nodes in the vector. The nodes themselves are accessed using the `TREE_VEC_ELT` macro, which takes two arguments. The first is the `TREE_VEC` in question; the second is an integer indicating which element in the vector is desired. The elements are indexed from zero.