r which the two item still belongs to the same tree. That k is returned,
and the same is done for these two items for the second tree.
There are n over 2 combinations of such pairs of items from the items in
the tree, and all of these numbers are calculated for each of the two trees.
Then, these two sets of numbers (a set for the items in each tree)
are paired according to the pairs of items compared, and a spearman
correlation is calculated.</p>
<p>The value can range between -1 to 1. With near 0 values me