private ExprNodeGenericFuncDesc getInExprNode(ExprNodeDesc en) {
      if (en == null) {
        return null;
      }

      if (en instanceof ExprNodeGenericFuncDesc && ((ExprNodeGenericFuncDesc)(en)).getGenericUDF()
          instanceof GenericUDFIn) {
        return (ExprNodeGenericFuncDesc) en;
      }
      return null;
    }
Return the expression node descriptor if the input expression node is a GenericUDFIn.
Else, return null.
@param en Expression Node Descriptor
@return The expression node descriptor if the input expression node represents an IN clause.
Else, return null.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/PartitionColumnsSeparator.java