I should have done it last week, but had to postpone when I unexpectedly spent last Monday in an ice storm in Munich airport and needed the rest of the week to catch up.
So I went to the dermatologist this morning, and she merrily stuck squares soaked in various substances to me, covering it all over with a huge bandage. She will check them on Wednesday and make a final assessment on Friday.
And I can't have a shower until Friday. I think it is decades since I went as long as four days without a shower. (Obviously I can wash by other means, but Thursday evening's posh reception will be interesting.)
And the worst of it, as I am now realising, is that the bandage is really itchy; it is deliberately placed in the most inaccessible part of my back, and in any case, if I actually am allergic to any of these things, of course it's going to bloody itch - until Friday, for the next four days!
I just hope I actually am allergic to something or other, to make the whole exercise worthwhile.