code: public static Supplier<Void> supplier(Runnable adaptee) {
          dbc.precondition(adaptee != null, "cannot adapt a null runnable");
          return () -> {
              adaptee.run();
              return null;
          };
      }
docs: Adapts a runnable to a supplier.
  
  @param adaptee the runnable to be adapted
  @return the adapted supplier