[dev-context] two proposals for attachments in PDF/A-3

[Pablo Rodriguez]

Hans,

I realized that the values for the /AFRelationship key that I provided
in a previous message, are the ones defined for PDF-2.0 (Source, Data,
Alternative, Supplement, EncryptedPayload, FormData, Schema or Unspecified).

According to
https://www.loc.gov/preservation/digital/formats/fdd/fdd000360.shtml,
values for the /AFRelationShip key in PDF/A-3 are only: Source, Data,
Alternative, Supplement, and Unspecified.

If you allow me a suggestion, maybe it would be useful to have two new
options in the \setupattachment command:

  relationship  =  source data alternative supplement unspecified

  association   =  document page

The default values could be document and unspecified. The user should
know what to do when changing these defaults.

Of course, the /AF entry can be placed in other PDF objects, but
document and page are enough options to start with.

Many thanks for your help,

Pablo

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