- (void) setOrder: (NSString*) fieldlist;

Function setOrder() sets order of records by some field or list of fields.
The function does not change a file, it re-orders records in a memory.
One function accepts a string with fields delimited by comma, another function accepts an array.
To return the original order of records - call unserOrder().
function descendingMode() set either ascending or descending order.

- (void) setOrderA: (NSArray <NSString*>*) array;
- (void) unsetOrder;
- (void) descendingMode:(BOOL)descending;

{
    // your code
    [dbf setOrder:@"FIRSTNAME,LASTNAME"];
    [dbf descendingMode:YES];
    [dbf setOrderA: @[@"FIRSTNAME", @"LASTNAME"] ];
    // your code
    [dbf unsetOrder];
}