TA1: generate Interchange Acknowledgement when ISA envelope errors detected

[azoner]

Background

pyx12 generates 997/999 transaction-level acknowledgements but does not currently generate TA1 — the X12 Interchange Acknowledgement that responds to ISA-level (envelope) problems. TA1 is sent in response to issues with the interchange itself (sender/receiver IDs, control numbers, envelope structure) rather than transaction-set content.

Specifics

Two TODO comments flagged this gap:

• `x12n_document.py:100` — `# XXX Generate TA1 if needed.` (at parser init, before main loop)
• `x12n_document.py:155` — `# XXX Generate TA1 if needed.` (at IEA-segment close, alongside the 997 generation comment)

These comments date from 2003 (~22 years ago) — they predate most of the project's modern history. No user has ever filed for them, so HIPAA-flow users (the project's primary audience) clearly don't need TA1. May matter for non-HIPAA X12 EDI use cases.

Scope

• Track interchange-level errors (envelope structure, ISA/IEA mismatches, control-number issues) separately from transaction errors
• On IEA close, if interchange errors were collected, emit a TA1 segment per X12 spec
• TA1 has its own envelope rules — implementer should consult the X12 standard (or 5010 implementation guide if scoping to HIPAA-adjacent flows)

Priority

Low — no user has requested this in 22 years. Tracking for completeness; close as wontfix if no demand surfaces.