alternatives()
  Alternative scanners.
  
  Returns a virus, if one scanner returns a virus.
  Scannig is ended if one scanner returns virus or clean.
  Next scanner from the list is used, if previous scannes fails
  (raises an error). Raises an ScannerError, if all scanners failed.
  
  Usage: alternatives(scanner1(),scanner2(),...)
  
  Where: scanner1(),... are scanners, which may find a virus