SYSML2 — Graphical notation for nested reference usage needs resolution

According to the Graphical BNF, the notation for a nested reference usage is now a white diamond in the upper-right hand corner of the usage shape (or optionally a black diamond for a non-reference usage, which is the default). In the Graphical BNF productions this is represented by rd = (reference-diamond)? as defined in clause 8.2.3.6. This is a change from the previous dashed-outline shape as also used in SysML v1.

There are arguments pro and con for which notation might be the more usable, including the affinity of the white diamond with a feature membership, but also continuity with SysML v1 and its visibility of the distinction around the whole shape.

The new notation was proposed primarily to avoid the practical difficulties providing dashed-outline versions of every usage shape in the BNF, but the notation should be properly decided on its own merits, not to make things easier for the BNF. An informal comment could be provided in the BNF simply stating that a dashed-outline version is available for each shape according to whether it is a reference usage. An additional alternative is to use dotted-outline, with the advantage that it more closely follows non-right-angled shapes, such as rounded rectangles used for usages.

After discussion the Graphical Specification WG recommends to replace the upper-right-hand corner reference-diamond notation with a dotted-outline for reference usage, and stay with a solid-outline for composite usage.

Graphical notation for nested reference usage needs resolution

The revised text reflects the usage of dashed boxes or ref <kind> keywords instead of hollow diamond adornments to denote referential usages. The dashed boxes are used in context of interconnection views, action flow and state flow views. It also removes erroneous inclusion of definition nodes in interconnection views. It is noted that dashed node notations are also used for namespace imports, however since reference nodes and imported nodes are specified in disjoint views, we regard this as a minor issue.

The motivation for this issue and its resolution is that the proposed dashed notation for referential usages is more visually prominent on diagrams and is consistence with the SysML v1 notation for reference properties. The general consensus of the FTF is that it was a mistake to introduce a significantly different notation than SysML v1 for purely technical reasons (as described in the issue description).