Once again, these ponderings made me realize something I've never before
realized in years and years of thinking about this stuff.

Dictionary definition of the noun "format":

* the way in which something is arranged or set out
* the shape, size, and presentation of a book or periodical
* the medium in which a sound recording is made available
* a defined structure for the processing, storage, or display of data

All of that says how to "physically" lay out stuff on a surface.

None of it says anything about thinking up rules that are not part of
the physical layout. Nothing about inspecting and filtering what stuff
can be laid out (the book and sound recording examples talk about limits
on the medium itself, which is indeed what also we do when we choose
between a uint32 or a uint64 for example; they do not imagine extra
limits on the contents once the physical structure has been laid out).

It's so simple. The format is the shape of things. Anything in a format
spec that doesn't lay out bytes is extraneous.