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This website is open and free. It started in July 2017 as an experiment to organize my lecture notes, exercises and tests for my IB students but it was quickly embraced by many students and teachers around the world.
Since 2017 I am continuously striving to revise, improve and almost recently attune this material to the new syllabus. The help of your comments is invaluable and makes me proceed with enthusiasm.
I am often urged by some fellows to introduce a small charge for all this effort and time consuming involvement. However, I strongly believe that even an elementary fee will be the beginning of the end for this fascinating route. I want to keep the site open and free as it started.
The idea of a donation was born last year by an international student who insisted to express his gratitude in this way. I kindly refused at that time. As this work tends to become a full time engagement I have decided to implement this option for those who wish to support, as an additional motivation for me to carry on and further enhance the content of this site.
Thank you all,