/**
  * Replaces an existing Conditional Formatting rule at position idx.
  * Excel allows to create up to 3 Conditional Formatting rules.
  * This method can be useful to modify existing  Conditional Formatting rules.
  *
  * @param idx position of the rule. Should be between 0 and 2.
  * @param cfRule - Conditional Formatting rule
  */
 public void SetRule(int idx, IConditionalFormattingRule cfRule)
 {
     XSSFConditionalFormattingRule xRule = (XSSFConditionalFormattingRule)cfRule;
     _cf.GetCfRuleArray(idx).Set(xRule.GetCTCfRule());
 }