SYSML2 — Constraints missing to enforce variations being abstract

In the SysML Specification, 7.6.7 Variations and Usages states “A variation is always abstract, so the abstract keyword is not used on a variation.” And, indeed, the textual notation BNF in 8.2.2.6.1 Definitions and 8.2.2.6.2 Usages allows either the abstract or variation keyword to be applied to a definition or usage declaration, but not both. However, there are no constraints in the abstract syntax subclauses 8.3.6.2 Definition or 8.3.6.4 Usage to enforce this.