readme: https://www.geogebra.org/m/w8uzjttg
  https://www.geogebra.org/m/gvn7m78g
  https://www.geogebra.org/m/arxecanq
  https://www.geogebra.org/m/xb69bvww
  https://www.geogebra.org/m/apvepfnd
  https://www.geogebra.org/m/evmj8ckk
  https://www.geogebra.org/m/qxcxwmhp
  https://www.geogebra.org/m/p3cxqh6c
  https://www.geogebra.org/m/ggrahbgd
  https://www.geogebra.org/m/pnhymrbc
  https://www.geogebra.org/m/zjukbtk9
  https://www.geogebra.org/m/bbezun8r
  https://www.geogebra.org/m/sgwamtru
  https://www.geogebra.org/m/fpunkxxp
  https://www.geogebra.org/m/acxebrr7