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I keep thinking about pcb's file (and thus data) format.  I have lots
of ideas but no traction :-(

Some ideas:

Elements should be able to reference a common footprint or copy it to
make a private footprint (sketchup does this a LOT).  I.e. all DIP16
elements should share a common footprint, and editing one should edit
them all.  Note we have the "inhereted attributes" problem all over
again, like gschem - the footprint provides a default refdes location
but nearly every element changes it, but that shouldn't mean needing a
private footprint definition for each element.

Likewise, footprints should reference a common set of padstacks,
rather than have N independent but identical padstacks.  There should
be a way of overriding part of a padstack without having a private
copy of it?

These "common" definitions should be somewhat independent of the layer
stackup (i.e. symbolic layer types) and map to the specific layer
stackup somehow.  This lets developers change the stackup without
worrying about the footprints, and maybe opens the way to flex cables
where different regions have different stackups.

PCB should have a concept of a "sublayout" (i.e. four identical input
channels).  Sublayouts should be movable, rotatable, go on either
board size, yet if you edit a common one, they all change.  Sublayouts
should somehow relate to heirarchical schematics, including some rules
about how to name/rename parts within.

Footprints should be a sublayout :-)

/me starts another blue sky document...