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Could you add a command-line option for ltl2tgba to invert the input formula before computing the automaton?

Done. https://gitlab.lrde.epita.fr/spot/spot/commit/7f21d3ff29ad3a513f261025c5 75e4676a14cc50

Great - many thanks!

Any opinion on what passing --negate twice should do?

A really good and nerdy one - twice the smiley face! :-) :-) Cheers, Victor.