a->b->c->d vs e<-f->g->h ... ??


. a b c d ab bc cd abc bcd abcd
a 0 0 0 0
b 1 0 0 0
c 1 0 1 0
d 1 0 1 1
ab 0 0 0 0 0 0
bc 1 0 0 0 0
cd 1 0 1 0
abc 0 0 0 0 0 0
bcd 1 0 0 0
abcd 0 0 0 0

??something about trying to visualize this (or just part of it...) in terms of edges of 4-simplex... ??of course go back and look at for example 3-simplex and 2-simplex cases...