def branch_order(tree_section):
'''Branching order of a tree section
The branching order is defined as the depth of the tree section.
Note:
The first level has branch order 1.
'''
node = tree_section[-1]
bo = sum(1 for _ in tr.iforking_point(node, tr.iupstream))
return bo - 1 if tr.is_forking_point(node) else bo
