Perhaps I have not been clear as to the problem I am having with this use of "is."
I understand "is" to mean one thing "is equal" to another thing.
OK, thanks for the explanation. But I still think that your example is not actually "equative" in this sense.
But does the use of "was" imply that same equality? "Was" implies change in at least one side of this 'equa'tion, which would have to be universal for the use of "was" to apply to both sides.
Or am I just mudying the effort?
Now I see what you mean, I hope.
There can be a bit of ambiguity once we start involving tense (also modals, though they were not part of your question).
I will substitute my own example, which I think is genuinely equative:
(1) Mr. Obama is the president of the United States.
(2) Mr. Clinton was the president of the United States. [in 2002, say]
In (2), we have an ambiguity, depending on what time the phrase "the president of the United States" refers to. If it refers to the time when Clinton was president, then it would be true ("in 2002, it was the case that Bill Clinton was equal to the individual who at that time was described as the president of the United States"). It could also mean (in principle) that Clinton was equal to the man now described as president of the United States, namely Obama (which would of course be false).
Nonetheless the copula is still trying to make an equative claim, so I wouldn't say that its equative nature is affected by involving tense. It is just that evaluating the truth or falsity of the sentence becomes more complicated.
I hope I have understood you correctly now.
If you (or anybody else reading this) is interested in reading more about this, it is often called the issue of "intensionality" (and no, that isn't a spelling mistake
).