int CallFunction(void *function, cFunctionData *attribute, void *data);

  CCTK_OverloadSyncGroupsByDirI(SyncGroupsByDirI);

  int didsync = 1;

  int didsync = 0;

}
int SyncGroupsByDirI(const cGH *restrict cctkGH, int numgroups,
                     const int *groups, const int *directions) {
  assert(cctkGH);
  assert(numgroups >= 0);
  assert(groups);
  // TODO: Synchronize only current time level. This probably requires
  // setting up different mfabs for each time level.
  for (int n = 0; n < numgroups; ++n) {
    int gi = groups[n];
    auto &restrict groupdata = ghext->groupdata.at(gi);
    // we always sync all directions
    groupdata.mfab.FillBoundary(ghext->geom.periodicity());
  }
  return numgroups; // number of groups synchronized

int SyncGroupsByDirI(const cGH *restrict cctkGH, int numgroups,
                     const int *groups, const int *directions);

  SYNC: phi

  SYNC: phi

SCHEDULE WaveToyAMReX_Sync AT poststep
{
  LANG: C
  OPTIONS: level
} "Synchronize wave equation state"

extern "C" void WaveToyAMReX_Sync(CCTK_ARGUMENTS) {
  DECLARE_CCTK_ARGUMENTS;
  DECLARE_CCTK_PARAMETERS;
  // Synchronize
  for (auto &restrict groupdata : ghext->groupdata)
    groupdata.mfab.FillBoundary(ghext->geom.periodicity());
}