Change cardinality of Relationship "to" property from 1..* to 0..*

[goneall]

If a Relationship has a completeness of noAssertion, there would not be an element for the to property.

This could be solved in one of three ways:

1. Have a special Element NoAssertion that would be referenced in the to property. This is similar to today's solution.
2. Change the cardinality of to from 1..* to 0..* and leave the completeness property as is (including a noAssertion value).
3. Change the cardinality of to from 1..* to 0..* and remove the noAssertion value from the completeness property since having a to with no elements would imply noAssertion.

My preference would be 2 above is it is the cleanest to implement. The only downside I can think of is having a noAssertion value and having one or more to elements doesn't seem to make sense.

Having a NoAssertion element in solution 1 above is quite challenging to implement in some computer languages and in RDF, as we learned in the SPDX 2.X implementations.

[seabass-labrax]

As I have mentioned during a recent SPDX Tech Team meeting, I think this is a bizarre and pooly-defined aspect of SPDX, and thus should not be included in the forthcoming SPDX 3.0 specification. I'm aware that some stakeholders (seemingly with affiliations related exclusively to computer security efforts in the USA) have expressed a desire for a 'known unknown' facility in Software Bill of Materials, but I have yet to see a concrete example of such a capability being used in reality. My preference would therefore be No. 3, as it at least allows Relationship Elements with empty 'To' fields to be ignored in processing without undue difficulty.

[kestewart]

Resolved in #293