https://hackmd.io/s/r14Do56HN exercise: https://hackmd.io/s/rJtAFuTSV
https://hackmd.io/s/Hy4B5cDL4# Solution: https://hackmd.io/s/rJsKrMxPV# exercise: https://hackmd.io/s/rJ19JTvUE
https://hackmd.io/s/SJ7nxwRL4# exercise: https://hackmd.io/s/r1N9lPC8N#