Mathematics and measurement assume that everything used in one equation equals the same quantities at the beginning of calculation or axiomatization as at the end. That means it is mathematically correct to say that the idea of "equal" means "equal from the time the process starts to the time it ends." In General Semantics and E Prime the words equal, remain (for the past until now) and become (for now into the future) replace the verb "to be" for this reason.