"silk is cloth" is not true except for the colloquial interpretation that infers "silk can sometimes be a cloth". Given the clarifications, it seems the OP is intending to say it's an exact equality and not a colloquial definition that actually means "subset".

I'll note, I have not asked any questions other than (paraphrasing): "what do you mean precisely?" To which, I have not gotten any answers other than trolling and flaming; and examples that all conveniently swap "is" with "is a".