  It is well known that in a binary tree the external path length minus the
internal path length is exactly 2n-2, where n is the number of external nodes.
We show that a generalization of the formula holds for compacted tries,
replacing the role of paths with the notion of extent, and the value 2n-2 with
the trie measure, an estimation of the number of bits that are necessary to
describe the trie.
