I have played with Isabelle before, but not intensely. I'm now writing one proof every day for 100 days to get a firmer grip. The first ones will be based on exercises and later ones will be more serious.
P1: simple stuff
P2-7, 13, 8, 14: 1.1-1.6, 1.7, 1.8, 1.9
P9-10: 3.1-3.2
P11-12, 15-17: 4.1-4.5
P18: 2.1
P19, 20: 2.2
P21: 2.3
P22: 2.4
P23: 2.5
P24-25: 5.1
P26: 5.2
Day 27: Automata/AutoProj, Automata/DFA
Short pause at the end of December 2020 due to travel. Resumed on Jan 1.
Day 28: Automata/NFA
Day 29: Automata/FA_N2D_EQUI (not finished)
Day 30: FunWithFunctions/identity1
Day 31: FunWithFunctions/identity2