
It is obviously too late at night for such an obscure reference to calculus, aka differentiation, and the other one what I can never remember.
I have spent two hours this evening amending a database, on a program that supposedly cannot read a database, then sent myself the wong copy of the email. Yes, I wanted to send myself a copy so that I know I can send someone else the copy so that they can print it.
Then I could not find the correct copy, and thought I had mis-named it.

Now found the right copy.

And emailed it.
Apart from that it was quite straightforward really.
