2-3 Trees
For "nodes" that contain two data items
there can be either no children or 3 children:
In this case, the value of the smaller key at the
"node" must be greater than the value of each
key in the left subtree and smaller than the
value of each key in the middle subtree.
The value of the larger key at the "node" must
be greater than the value of each key in the
middle subtree and smaller than the value of
each key in the right subtree.