steps to reproduce

1. load the attached excel file
2. delete row 15
3. delete row 17

current result

Sep 26, 2016 12:55:59 PM org.zkoss.zk.ui.impl.UiEngineImpl handleError
SEVERE:
java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:394)
at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:405)
at org.zkoss.zss.model.impl.FormulaTunerHelper.shrinkDataValidationRef(FormulaTunerHelper.java:1018)
at org.zkoss.zss.model.impl.FormulaTunerHelper.shrink(FormulaTunerHelper.java:880)
at org.zkoss.zss.model.impl.SheetImpl.shrinkFormula(SheetImpl.java:1455)
at org.zkoss.zss.model.impl.SheetImpl.shiftAfterRowDelete(SheetImpl.java:659)
at org.zkoss.zss.model.impl.SheetImpl.deleteRow(SheetImpl.java:610)
at org.zkoss.zss.range.impl.InsertDeleteHelper.delete(InsertDeleteHelper.java:317)
at org.zkoss.zss.range.impl.RangeImpl$15.doInvoke(RangeImpl.java:936)
at org.zkoss.zss.range.impl.RangeImpl$ModelManipulationTask.invoke(RangeImpl.java:1746)
at org.zkoss.zss.model.util.ReadWriteTask.doInWriteLock(ReadWriteTask.java:43)
at org.zkoss.zss.range.impl.RangeImpl.delete(RangeImpl.java:939)
at org.zkoss.zss.api.impl.RangeImpl.delete(RangeImpl.java:442)
at org.zkoss.zss.api.CellOperationUtil.delete(CellOperationUtil.java:902)
at org.zkoss.zss.ui.impl.undo.DeleteCellAction.doAction(DeleteCellAction.java:61)