NS_ENSURE_TRUE(mRules, NS_ERROR_FAILURE);
if (!mRules)  { return NS_ERROR_DOM_INDEX_SIZE_ERR; }
CSSMediaRuleImpl::DeleteRule(PRUint32 aIndex){  NS_ENSURE_TRUE(mSheet, NS_ERROR_FAILURE);  NS_ENSURE_TRUE(mRules, NS_ERROR_FAILURE);    PRUint32 count;  mRules->Count(&count);  if (aIndex >= count)    return NS_ERROR_DOM_INDEX_SIZE_ERR;  return mSheet->DeleteRuleFromGroup(this, aIndex);}