SYSML21 — Subject of a subrequirement

In 7.21.2 Requirement Definition and Usage, in the second to last paragraph, it states:

...if a subject parameter is not explicitly declared for a subrequirement, it is assumed to have the same subject as its containing requirement definition or usage, with its subject bound to that of the container.

However, there is no constraint in the abstract syntax or the model library to enforce this.