Oh, s***.
Is it a good question? Good question! For me the question was: What exactly is the question?!
Well, I already had troubles to solve the SQL arithmetic, I therefore wasn't aware of the troubles arising when using a view in an INSTEAD OF trigger...
And I am still struggling with it, even with Hugos' explanation...
🙁
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If you set out to do something, something else must be done first.