@Burr
I follow your explanation but seems with this file I must miss something! :)

http://moiscript.weebly.com/uploads/3/9/3/8/3938813/b_solution.3dm

<< Something like Anchor constraints need to be applied to use the basic snap mechanics.
Yes! In this case double anchors! (or maybe a very tricky geometric construction - that i wish of course! ;)
(geometric because mathematical is not yet very cumfortable for my very old neurons! :)