substructures and aliases

7.28: adde/alias handling:
. an alias is dangling when its target is deleted;
so, any delete operation should involve asking
whether aliases should be deleted also;
or, whether their appearance should be changed to reflect the dangling state:
eg, arrow symbols are changed to null, and name greyed .
. copying across volumes will copy the link .
. adde should also make a backup of link`targets;
and, also address complications of
duplications, and sharing updates, etc,
ie, when copying across volumes .
8.3: links are of 2 types:
. one is invisible so that any copy to the unit
is going to copy what is pointed at by the invisible link .
. the visible link is an url that means it is external to the unit;
and so should be copied only if wanting to copy multiple units .
. hence there needs to be some way to describe what a unit is;
like git identifies a project as the unit .

7.28: adde/subtructuring with modular skeleton:
. an efficient way to modularize a file
is to put the structure of it
(the table of contents with links to pararaphs)
in one file (the skeleton subfile)
so that changing a subtitle opens just the skeleton subfile .