Comment (by Roland Haas):

FLRWSolver is included in the ET as of the 2022_11 Kowalevski release: https://www.einsteintoolkit.org/about/releases/ET_2022_11_announcement.html

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2549/inlcude-flrwsolver-in-et