x relationship ->: e.g. on evidence page, get claims it supports
for block x (e.g. current page)
x
for every block r that contains a page reference to x
r
get child block c of r
c
if c contains -> AND a page reference to [[relationship]]
->
[[relationship]]
return all children of c
get parent block p of r
p
if r contains <- AND a page reference to [[relationship]]
<-
return parent of p