1112(undefined) may be useful with payload

[chrysn]

Processing packed-by-reference#5, I noticed that 1112(undefined) can cause problems:

• When an application compares unprocessable elements for identity, 1112(undefined) will show up as identical. (Granted, "don't do that, then")
• If an unpacker has two unpopulated entries in a single dict's keys, and thus produces two 1112(undefined), all of a sudden the dictionary has duplicate keys.

[chrysn]

Possible mitigation would be to allow decoder defined payload in the 1112 (which would be ignored by the recipient but is stuctured as the decoder needs it), or doing away with 1112 altogether and instead leaving the references as they are (resulting, once more, in a tag that is not processable by the recipient, but it is already a bit disambiguated).

Either of those solves dictionary key issue. The compare-by-identity issue is just mitigated a bit, but not solved for good, because when comparing across positions in different documents, 1112's payload would need to encode everything the decoder knows about the chain of tables that led to this, which is very verbose.