[Mp4-tech] Complementary non-reference pairs in H.264

[i.g.richardson rgu.ac.uk]

Hello
I posted this message to JVT-Experts but haven't yet had a reply, so I am trying again..
Is it possible for a complementary non-reference field pair to contain 2 fields from successive frames (bottom field of frame 1, top field of frame 2) ? This does not appear to be excluded as a possibility in the definition in H.264 (section 3); e.g. if the top field of frame 1 was marked for reference and the bottom field of frame 1 was not, then the pair could (in theory) be formed from bottom field of frame 1 + top field of frame 2.
This could happen if (say) nal_ref_idc is not equal to zero for top field of frame 1 but is equal to zero for bottom field of frame 1 and top field of frame 2. I haven't (yet) found anything that precludes this possibility. 
I would be grateful for advice as to whether this interpretation is correct.
Many thanks
Iain Richardson
HYPERLINK "mailto:i.g.richardson rgu.ac.uk"i.g.richardson rgu.ac.uk
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