cdm/src/main/java/ucar/nc2/NetcdfFileWriter.java
public Structure addRecordStructure() {
    if (version != Version.netcdf3) return null;
    boolean ok = (Boolean) ncfile.sendIospMessage(NetcdfFile.IOSP_MESSAGE_ADD_RECORD_STRUCTURE);
    if (!ok)
      throw new IllegalStateException("can't add record variable");
    return (Structure) ncfile.findVariable("record");
  }
public Structure addRecordStructure() {
    if (version != Version.netcdf3) return null;
    boolean ok = (Boolean) ncfile.sendIospMessage(NetcdfFile.IOSP_MESSAGE_ADD_RECORD_STRUCTURE);
    if (!ok)
      throw new IllegalStateException("can't add record variable");
    return (Structure) ncfile.findVariable("record");
  }
For netcdf3 only, take all unlimited variables and make them into a structure.
@return the record Structure, or null if not done.