SYSML21 — Nesting of lifelines in sequence views not supported

Currently, the graphical notation does not support nesting of the head node of a lifeline in sequence views. See subclause 8.2.3.9, Graphical BNF production for lifeline with &sq-head-node.

It should be possible to represent nested occurrences under an owning occurrence as a &sq-head-node, where each of the nested occurrences have their own lifeline. A similar capability exists in SysML v1.