readme: https://www.geogebra.org/m/bbuczchu
  https://www.geogebra.org/m/xwyasqje
  https://www.geogebra.org/m/mx2cqkwr
  https://www.geogebra.org/m/tkqqqthm
  https://www.geogebra.org/m/asdaf9mj
  https://www.geogebra.org/m/ywuaj7p5
  https://www.geogebra.org/m/jkfkayj3
  https://www.geogebra.org/m/hptnn7ar
  https://www.geogebra.org/m/de9cwmrf
  https://www.geogebra.org/m/yjc5hdep
  https://www.geogebra.org/m/nm8r56w5
  https://www.geogebra.org/m/j7wfcpxj