FormLetter class COC method in D365 X++

[ExtensionOf(classStr(PurchFormLetter))]
final class PurchFormLetter_Extension
{
    static container checkParmTable(Num      parmId,
                                           boolean  vendAccountInQuery,
                                           boolean  invoiceAccountInQuery)
    {
        PurchParmTable                  purchParmTable;
        PurchParmLine                    purchParmLine;
        PurchLine                            purchLine
        Container                             loc;
        loc                                        = next checkParmTable(parmId,vendAccountInQuery,invoiceAccountInQuery);

        select PurchId from purchParmTable
            where purchParmTable.ParmId == ParmId;

        select ReqPlanIdSched from purchLine
            where purchLine.PurchId == purchParmTable.PurchId
            && purchLine.ReqPlanIdSched != "";
        //Do rest of the activities
     }
}