* \properties \em mlt_type the name of the service subclass, e.g. mlt_producer
* \properties \em mlt_service the name of a producer subclass
* \properties \em _position the current position of the play head, relative to the in point
* \properties \em mlt_type the name of the service subclass, e.g. mlt_producer
* \properties \em mlt_service the name of a producer subclass
* \properties \em _position the current position of the play head, relative to the in point