Comments on SIP distortion FITS convention

I have looked at the submitted document. I had also a quick glance to
WCS Paper IV (by the way, why is it still in draft status since such a
long time ?)

The document on the SIP convention fulfills the requirements for
inclusion in the registry (the document is clear enough and the
convention has been in real usage and is already supported by commonly
used software).

Although I'm not an expert at all on distortions, my impression is that
the SIP convention is definitely simpler, while WCS Paper IV is more
general.

So my concern would be about the coexistence of two partially
overlapping conventions ... of course this depends on whether WCS Paper
IV progresses toward publication (and if it does, it is likely it will
become part of the standard as the other WCS papers). What are the plans?
Could it incorporate the SIP convention in some compatibility mode ?

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