Reminds me of the math lectures during university:

"Since we all know that ... it is obvious that ..."

and I kept thinking "no we don't and no it isn't"

Those are just stepping stones to "It can be shown" and "Left as an exercise for the reader."

Thank you for posting this comment. I intended "we all know" as a device, an "obvious" counterfactual to emphasize the point, that there's this bizarre gap in what's taught. I wasn't quite happy with it, and later worried it might burn someone ("I guess it's just me that doesn't know that"), but it was late, and I punted. So, my thanks for the save.