code: public void addCommand(String key, Command<? super T> handler) {
      commands.put(key, handler);
    }
docs: Adds a new command handler.
  @param key The <code>String</code> that identifies this command.  The
  command may be triggered by a command line argument with this
  value.
  @param handler The <code>Command</code> that is used to process the
  command.