for (int vi = 0; vi < groupdata.numvars; ++vi)
              assert(coarsegroupdata.valid.at(tl).at(vi).valid_int &&
                     coarsegroupdata.valid.at(tl).at(vi).valid_outer &&
                     coarsegroupdata.valid.at(tl).at(vi).valid_ghosts &&
                     groupdata.valid.at(tl).at(vi).valid_int);

            bool some_sources_invalid = false;
            for (int vi = 0; vi < groupdata.numvars; ++vi) {
              if (!coarsegroupdata.valid.at(tl).at(vi).valid_int) {
                CCTK_VWARN(CCTK_WARN_ALERT,
                  "Prolongation requires valid interior in coarse grid (level = %d), but found it invalid for '%s' on timelevel %d",
                  level - 1, CCTK_FullVarName(vi), tl);
                some_sources_invalid = true;
              }
              if (!coarsegroupdata.valid.at(tl).at(vi).valid_outer) {
                CCTK_VWARN(CCTK_WARN_ALERT,
                  "Prolongation requires valid boundary in coarse grid (level = %d), but found it invalid for '%s' on timelevel %d",
                  level - 1, CCTK_FullVarName(vi), tl);
                some_sources_invalid = true;
              }
              if (!coarsegroupdata.valid.at(tl).at(vi).valid_ghosts) {
                CCTK_VWARN(CCTK_WARN_ALERT,
                  "Prolongation requires valid ghosts in coarse grid (level = %d), but found it invalid for '%s' on timelevel %d",
                  level - 1, CCTK_FullVarName(vi), tl);
                some_sources_invalid = true;
              }
              if (!groupdata.valid.at(tl).at(vi).valid_int) {
                CCTK_VWARN(CCTK_WARN_ALERT,
                  "Prolongation expects valid interior in fine grid (level = %d), but found it invalid for '%s' on timelevel %d",
                  level, CCTK_FullVarName(vi), tl);
                some_sources_invalid = true;
              }
            }
            if (some_sources_invalid)
              CCTK_ERROR("Some sources for prolongation were invalid");