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.