A simple balanced tree.
The struct simple-tree has a constructor that takes the rank and optionally the depth as parameters.
Accessor simple-tree-index of type fixnum points to the current node in simple-tree-nodes.
Read-only accessor simple-tree-nodes of type simple-vector refers to the nodes or leaves.
Read-only accessor simple-tree-depth of type fixnum refers to the depth of the tree.
Read-only accessor simple-tree-rank of type fixnum refers to the rank of the tree.