more general, somewhat less ugly containing address selector
more general, somewhat less ugly containing address selector

It can be surprising and discouraging to notice how much effort it takes
to discover the right abstractions you need to formulate your functions
in a concise and lucid way; our barely-comprehensible
`containing-address-oriented` here is an improvement over the previous
incomprehensible and specialized `containing-address-down`, but I expect
we have some ways to go before we have a function we can be proud to
use.

zackmdavis/xrosslexior