SYSML2_ — Correction to the resolution of SYSML2_-510

At one point in the drafting of resolution SYSML2_-511 to issue SYSML2_-510, the derivation of Usage::mayTimeVary was such that variation usages were not variable. However, this is not the case in the final resolution as approved. Unfortunately, the approved resolution still calls for the adding of the following statement to 7.6.7, which is no longer correct:

A variation is never time-varying (see 7.9.2), so the constant keyword is also not used on a variation.

Consistent with this, the update to the production RefPrefix in 8.2.2.6.2 also disallows using the keywords constant and variation together, which it shouldn't.

Remove incorrect statement

The incorrect statement in 7.6.7 should be removed and the production RefPrefix revised to allow the keywords constant and variation to be used together.