* To whom all correspondence should be addressed. ( (012) 420-3265; fax(012) 420-3266; e-mail mikro2@scientia.up.za Received 25 November 1997; accepted in revised form 15 October 1998.
* To whom all correspondence should be addressed.( (012) 420-3265; fax(012) 420-3266; e-mail mikro2@scientia.up.zaReceived 25 November 1997; accepted in revised form 15 October 1998.