This paper fixes invariance as a structural condition. The upper bound is fixed as a structural bound of ordering and is denoted as c. The bound is not speed, propagation, or rate, and is not measurable or comparable. Invariance is not constancy or equality. It is fixed as non-variability. The upper bound does not vary and is not parameterized, measured, or observed. No subject is introduced. No meaning is introduced. No comparison or temporal persistence is assumed. No causality is introduced. The system is not extended or derived. Invariance is fixed as the structural condition of the upper bound as non-variable.
Juza Minamikata (Mon,) studied this question.