first there 'd been my uniform to buy - a green jumper , light green shirt , green tie , grey trousers , black shoes - then books , notepaper and a4 writing pads , a ruler , pens and pencils , an eraser , set squares and a compass , as well as a scientific calculator , whose array of strange buttons - 'inv ' , 'sin ' , 'cos ' , 'ee ' - meant nothing to me .