Specman e: How the predefined sequence.item should be used?

I have a sequence:

sequence bus_sequence using item=bus_item, created_driver=bus_sequence_driver;

When I try to drive an item using the next code:

    extend bus_sequence {        
        body() @driver.clock is only {
            do item;
        };
    };

I get a compilation error that 'bus_sequence' does not have a field 'item'. But when I add the field item to the bus_sequence:

    extend bus_sequence {

        !item : bus_item;

        body() @driver.clock is only {
            do item;
        };
    };

I get an error that Field 'item' was previously definedin the sequence statement line... How can I approach the predefined item field? Thank you for your help

The predefined item field belongs to the SIMPLE bus_sequence subtype, so it can neither be added to the base type, nor be accessed via it. And a given sequence may be of a kind other than SIMPLE.