Take inspiration from test structure to refactor ipd and presentation time bullet

[nigelmegitt]

https://github.com/w3c/imsc-hrm/blob/0be06f8788083fdb407ab19e9b3a0c2fff94c1a7/spec/imsc-hrm.html#L505-L507

This sentence has always been editorially problematic. It's substantively correct, but hard to read and parse. We had some conversations about it before and tried to improve it.

Noticing that w3c/imsc-hrm-tests#1 introduces a set of tests preceded dur... and another set preceded ipd... I wonder if we can take inspiration from that to refactor the specification including the later lines:

https://github.com/w3c/imsc-hrm/blob/0be06f8788083fdb407ab19e9b3a0c2fff94c1a7/spec/imsc-hrm.html#L572-L573

so that we set two distinct requirements, each of which maps to the corresponding tests. The idea is to make it clearer and easier to read - if that doesn't work, we shouldn't make a change.

The result could be two statements along the lines of:

• It SHALL be an error if DUR(En) is greater than IPD.
• It SHALL be an error if DUR(En) is greater than the the presentation time of En minus that of Em where m is the largest non-zero value that is both less than n and is such that Em is not an empty ISD

I would not support any such change that is substantive. i.e. not purely editorial at this stage of the Rec track process.

[nigelmegitt]

Propose to defer this to a future version given our publication timeline intentions and that this proposal changes, albeit editorially only, substantive normative text in the specification.