drop()
  Interscanner to drop viruses/spams. By default they are rejected.
  
  This scanner can be used to drop some viruses, like Klez.
  Klez sends fake sender address and this virus can't be sent
  back to sender, because sender is faked.
  
  Usage: drop(drop_pattern, scanners)
  
  This scanner does nothing else, like sets the DROP flag.
  Email is passed to parent scanners and is dropped in sagator.
  
  Two constants drop.DEFAULT and drop.DEFAULT_EXT (extensible version)
  can be used to drop most of worms, trojans and phishings, which fakes
  sender email addresses.
  
  Examples: drop(drop.DEFAULT, scanners)
            drop(drop.DEFAULT_EXT % 'SPAM|OtherVirus', scanners)
            drop('.', scanners) # drop every virus/spam