grib/src/main/java/ucar/nc2/grib/collection/PartitionCollectionMutable.java
@Nullable
    public GribCollectionMutable makeGribCollection() {
      GribCollectionMutable result = GribCdmIndex.openMutableGCFromIndex(dcm.getIndexFilename(GribCdmIndex.NCX_SUFFIX), config, false, true, logger);
      if (result == null) {
        logger.error("Failed on openMutableGCFromIndex {}", dcm.getIndexFilename(GribCdmIndex.NCX_SUFFIX));
        return null;
      }
      lastModified = result.lastModified;
      fileSize = result.fileSize;
      if (result.masterRuntime != null)
        partitionDate = result.masterRuntime.getFirstDate();
      return result;
    }
@Nullable
    public GribCollectionMutable makeGribCollection() {
      GribCollectionMutable result = GribCdmIndex.openMutableGCFromIndex(dcm.getIndexFilename(GribCdmIndex.NCX_SUFFIX), config, false, true, logger);
      if (result == null) {
        logger.error("Failed on openMutableGCFromIndex {}", dcm.getIndexFilename(GribCdmIndex.NCX_SUFFIX));
        return null;
      }
      lastModified = result.lastModified;
      fileSize = result.fileSize;
      if (result.masterRuntime != null)
        partitionDate = result.masterRuntime.getFirstDate();
      return result;
    }
the children must already exist