This observation is subject to the counter-observation that the backflow-edge tail could be adjacent to only to flow-edge heads:
This counter-observation is correct but not important because both edges are part of the same backflow, and the same observation can be made about BC's tail. The same counter-observation could then be made against BC's tail with one of two results: either the counter-observation's wrong and BC's tail is adjacent to a flow-edge tail or it's correct, in which case the claim advances one more edge. Eventually, however, the counter-observation fails because, if advanced far enough along the general flow path, the claim will apply to the sink node, which is adjacent to the out-flow pipe.
This page last modified on 24 January 2006.