eco subset part of in branching relationship with main stem of In the tree T depicted in https://oborel.github.io/obo-relations/branching_part_of.png, S is the main stem of T. There are no other main stems. If we were to slice off S to get a new tree T', rooted at the root of B1, then B1 would be the main stem of T'. x main_stem_of y if y is a branching structure and x is a channel that traces a linear path through y, such that x has higher capacity than any other such path. requires discussion Christopher J. Mungall