cripu = x1 is a bridge/span over/across x2 between x3 and x4 [unordered, typically destination first].Now if you say {farna} derivative can do what cripu3 and cripu4 can do then why do we have cripu3 and cripu4?When using cripu3/4 we can't even be sure where is destination due to "typically".
Also I don't understand how to use {relselfa'a} since it doesn't manage cripu3/4 that continue to exist possible creating conflicts with {relselfa'a}.