Detailed sub-issues for the topic of refactoring "sources"

Here is the promised follow-up to the email with the concise high-level proposal statement for refactoring sources.

This is a stab at identifying several of the key sub-issues that need to be discussed/decided to make the proposed approach a practical reality.

More detailed sub-issues to be discussed/decided:

What should the structure of the Reference TLO look like?

Current proposal is three properties: (this addresses the “external_ids” issue
 as well)

“reference_URL” (optional) - specifies a URL to the external reference

“external_identifier” (optional) - specifies an identifier for the external reference content

“defining_context” (optional) - specifies the context within which the external_identifier is defined (system, registry, organization, etc.)

Use “created_by_ref” on each TLO as shorthand for “Producer” source relationships?

Should “created_by_ref” shorthand and “Producer” Has Source relationship both be explicitly supported? 
What if there is conflicting information?

Should “created_by_ref” be optional or required?

Does leaving out both a created_by_ref and has_source relationship for an object imply anonymity?

What values should be in the default vocabulary for the Roles property on Has Source relationships?

Should “Has Source” relationship support many-to-one capability to assert lots of STIX content has the same source in an efficient manner? Is the “efficiency” issue necessary to address at this time?

Basically this:

Relationship

ID = id-5

From = id-2, id-3, id-4

To = id-1

Nature = Has Source

Versus this:

Relationship

ID = id-5

From = id-2

To = id-1

Nature = Has Source

Relationship

ID = id-6

From = id-3

To = id-1

Nature = Has Source

Relationship

ID = id-7

From = id-4

To = id-1

Nature = Has Source