This function calculates the cherry index ChI(T) for a
given rooted tree T. The tree must not necessarily be binary.
ChI(T) is defined as the number of cherries in the tree. A cherry
is a pair of leaves that have the same direct ancestor. Note, if a vertex u
has x leaves as direct descendants, the number of cherries induced by u is
The cherry index does not fulfill the definition of an (im)balance index given in "Tree balance indices: a comprehensive survey" (Fischer et al., 2021).
A rooted tree in phylo format.
cherryI returns the cherry index of the given tree.
A. McKenzie and M. Steel. Distributions of cherries for two models of trees. Mathematical Biosciences, 164(1):81-92, 2000. doi: 10.1016/s0025-5564(99)00060-7.
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