[tlaplus] Question about a TLA derivation

[Andrew Helwer <andrew.helwer@xxxxxxxxx>]

On page 68 of A Science of Concurrent Programs we have the following derivation:

□◇□F ≡ ¬◇¬¬□¬¬◇¬F ≡ ¬◇□◇¬F ≡ ¬□◇¬F ≡ ¬¬◇¬¬□¬¬F ≡ ◇□F

I am having trouble understanding this step in particular:

¬◇□◇¬F ≡ ¬□◇¬F

How is this step done? I remember getting stuck on this exact transformation the last time this was discussed a few months ago. Thanks!

Andrew Helwer

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