A method and system of carrying out garbage collection in a computer system. Individual upright passages through the body corresponding in number to the metered streams from the tank receive the gravitating product streams and direct each stream into either or both of an upper loading zone and a lower loading zone in the passage. If the count exceeds a certain value, then the monitoring system enters a secure state. For example, the surgeon may desire a change in at least one of the variables, e. The connecting section includes a first tubular body fitted around the main body. In the event of slipping of the engine-driven wheels, the system increases the engine output so as to increase the driving force of the electric-motor-driven wheels for increasing the total driving force made up of the engine-driven-wheel driving force and the electric-motor-driven-wheel driving force, thereby improving acceleration performance, unlike conventional techniques.