Comment (by Miguel Zilhão):

I don’t remember the exact details, but if I recall correctly there was an error or warning if this parameter was not extended? I could be wrong, though. In any case, I believe we just copied the behaviour from ML_BSSN and extended it trivially. Indeed, we never really intended that any other choice for the lapse evolution was used. If having such a choice is important, we can try to implement something.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2487/_evolution_method-leanbssnmol-in-lean-does