SYSML21 — Provide notation for guards on object flows

SysML2 should support both graphical and textual syntax for guards on flows. The graphical and textual syntax should be like the syntax for successions. Ideally, the abstract syntax created by parsing the graphical and textual syntax for guards on flows should be more direct than the approach suggested for translating SysML1 object flows into SysML2 (proposed in Section 7.7.3.3.50 of the SysML v1 to SysML v2 Transformation specification).