CCTK_OverloadEnableGroupStorage(EnableGroupStorage);
  CCTK_OverloadDisableGroupStorage(DisableGroupStorage);
  CCTK_OverloadGroupStorageIncrease(GroupStorageIncrease);
  CCTK_OverloadGroupStorageDecrease(GroupStorageDecrease);

int GroupStorageCrease(const cGH *cctkGH, int n_groups, const int *groups,
                       const int *requested_tls, int *status, const bool inc);

// storage handling
namespace {
int GroupStorageCrease(const cGH *cctkGH, int n_groups, const int *groups,
                       const int *requested_tls, int *status, const bool inc) {
  DECLARE_CCTK_PARAMETERS;
  assert(cctkGH);
  assert(n_groups >= 0);
  assert(groups);
  assert(requested_tls);
  for (int n = 0; n < n_groups; ++n) {
    if (groups[n] < 0 or groups[n] >= CCTK_NumGroups()) {
      CCTK_VWarn(1, __LINE__, __FILE__, CCTK_THORNSTRING,
                 "Group index %d is illegal", groups[n]);
      return -1;
    }
    assert(groups[n] >= 0 and groups[n] < CCTK_NumGroups());
    assert(requested_tls[n] >= 0 or requested_tls[n] == -1);
  }
  // sanitize list of requested timelevels
  std::vector<int> tls(n_groups);
  for (int n = 0; n < n_groups; ++n) {
    int ntls = requested_tls[n];
    int const declared_tls = CCTK_DeclaredTimeLevelsGI(groups[n]);
    if (inc and declared_tls < 2 and ntls > declared_tls) {
      char *groupname = CCTK_GroupName(groups[n]);
      CCTK_VWarn(CCTK_WARN_ALERT, __LINE__, __FILE__, CCTK_THORNSTRING,
                 "Attempting to activate %d timelevels for group '%s' which "
                 "only has a single timelevel declared in interface.ccl. "
                 "Please declared at least 2 timelevels in interface.ccl to "
                 "allow more timelevels to be created at runtime.",
                 ntls, groupname);
      free(groupname);
      ntls = declared_tls;
    }
    if (ntls == -1) {
      ntls = declared_tls;
    }
    tls.at(n) = ntls;
  }

  // TODO: actually do something
  int min_num_timelevels = INT_MAX;
  for (int n = 0; n < n_groups; ++n) {
    int const gid = groups[n];
    cGroup group;
    int ierr = CCTK_GroupData(gid, &group);
    assert(not ierr);
    // Record previous number of allocated time levels
    if (status) {
      // Note: This remembers only the last level
      status[n] = group.numtimelevels;
    }
   // Record (minimum of) current number of time levels
   min_num_timelevels = min(min_num_timelevels, group.numtimelevels);
  } // for n
  if (min_num_timelevels == INT_MAX) {
    min_num_timelevels = 0;
  }
  return min_num_timelevels;
}
}
int GroupStorageIncrease(const cGH *cctkGH, int n_groups, const int *groups,
                         const int *tls, int *status) {
  DECLARE_CCTK_PARAMETERS
  return GroupStorageCrease(cctkGH, n_groups, groups, tls, status, true);
}
int GroupStorageDecrease(const cGH *cctkGH, int n_groups, const int *groups,
                         const int *tls, int *status) {
  DECLARE_CCTK_PARAMETERS
  return GroupStorageCrease(cctkGH, n_groups, groups, tls, status, false);
}
int EnableGroupStorage(const cGH *cctkGH, const char *groupname) {
  const int group = CCTK_GroupIndex(groupname);
  assert(group >= 0 and group < CCTK_NumGroups());
  // TODO: decide whether to use CCTK_MaxActiveTimeLevelsGI
  const int tls = CCTK_DeclaredTimeLevelsGI(group);
  int status;
  GroupStorageIncrease(cctkGH, 1, &group, &tls, &status);
  // Return whether storage was allocated previously
  return status;
}
int DisableGroupStorage(const cGH *cctkGH, const char *groupname) {
  const int group = CCTK_GroupIndex(groupname);
  assert(group >= 0 and group < CCTK_NumGroups());
  const int tls = 0;
  int status;
  GroupStorageDecrease(cctkGH, 1, &group, &tls, &status);
  // Return whether storage was allocated previously
  return status;
}

int GroupStorageIncrease(const cGH *cctkGH, int n_groups, const int *groups,
                         const int *tls, int *status);
int GroupStorageDecrease(const cGH *cctkGH, int n_groups, const int *groups,
                         const int *tls, int *status);
int EnableGroupStorage(const cGH *cctkGH, const char *groupname);
int DisableGroupStorage(const cGH *cctkGH, const char *groupname);