The thing with the erasers was once you erased, the chemical was still on the paper so you could not re-write ont he erased parts. This meant you had to use the other-end of the eraser pen where there was a special blue ink pen. Often this would then "bleed" heavily (perhaps I didn't wait for the paper to dry?) and never looked the same as the fountain pen ink, so it was hugely obvious where you made a mistake as your normal wiring all looked normal, then you'd get this huge blurry fuzzy blue mess where you "erased" a mistake :-)