Comment (by Roland Haas):

I don’t think there is anything actually to do here. CartGrid3D scheduling its ApplyBCs routine in Boundary’s group becomes a no-op of thorn Boundary is not running (as intended). It does not introduce a dependency, between the two (at least for “old” uses of CartGrid3D that did not use Boundary before that thorn came into existence). The aliased function Boundary_SelectedGVs from thorn Boundary is used in only those scheduled functions which only run if Boundary is on. Similarly SymmetryRegister and SymmetryRegisterGrid are in group SymmetryRegister scheduled by SymBase (which also provides those functions).

So, I do not actually see any dependency and would content that the thorn is fine as is.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/931/cartgrid3ds-dependency-on-boundary