code: def transmute_sites( self, old_site_label, new_site_label, n_sites_to_change ):
          
          selected_sites = self.select_sites( old_site_label )
          for site in random.sample( selected_sites, n_sites_to_change ):
              site.label = new_site_label
          self.site_labels = set( [ site.label for site in self.sites ] )
docs: Selects a random subset of sites with a specific label and gives them a different label.
  
          Args:
              old_site_label (String or List(String)): Site label(s) of the sites to be modified..
              new_site_label (String):                 Site label to be applied to the modified sites.
              n_sites_to_change (Int):                 Number of sites to modify.
  
          Returns:
              None