docs: protected EscherAggregate getEscherAggregate(HSSFSheet sheet) {
          EscherAggregate agg = sheetToEscherAggregate.get(sheet.getSheetName());
          if (agg == null) {
              agg = sheet.getDrawingEscherAggregate();
              sheetToEscherAggregate.put(sheet.getSheetName(), agg);
          }
          return agg;
      }