Comment (by Miguel Zilhão):

i’ve checked Roland’s proposal above and it indeed seems to work fine for non-Llama evolutions. the current development version of Lean does support Llama, though, so it’d be good to cover that as well. i see that there is also the functionMultiPatch_GetBoundarySpecification, which i guess could be used when Llama is used?

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2288/leans-boundary-condition-setting-for-bssn