Comment (by Erik Schnetter):

A belated comment on the implementation: There could be global variables, one per grid function, that are initialized to -1, and which are set by the flesh for all active thorns. This way not extra functions need to be scheduled, and thorns can use these global variables to look up variable indices.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2319/do-not-use-static-variables-in