A slewing bearing having two rings and rollers arranged between the rings, wherein one of the rings is provided with a through-hole for inserting the rollers for assembly and maintenance, and with a plug for closing the through-hole. The electrode is coupled to a high frequency power supply to ablatively heat the urethra and the prostatic tissue near the urethra. A partial matrix inversion of the diagonal elements of the first product is generated to provide a diagonal vector.