Also, here's a maths puzzle for π-day:

Adding the brackets from this Alex Bellos column theguardian.com/science/2019/m to the numbers 1, 2, 3, 4 —

[1, 2] → 12
⟨1, 2⟩ → 21
⟨[⟨1, 2⟩, 3], 4⟩ → ⟨[21, 3], 4⟩ → ⟨213, 4⟩→ 4213

…we can make any number except 2413, and which other?

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