What's your stance on writing in the margins of books you own?

on non-fiction, and especially school textbooks. mostly stuff like "thrash", "?", "hassiktir lan ordan" when i disagree with the author. I often do because most of what I have to read are politic books mostly under influence of marxism

I once wrote a poem on the back of my Hamlet when I was 17. It was pretty bad, I think the opening was: "Certainly certainty is just a dream/ and dream just a fickle scene"