   @Test
   public void clearGroup() {

      Group<String, String> group = new Group<String, String>();
      group.addToGroup("a", "b");
      group.addToGroup("a", "c");
      group.addToGroup("b", "b");
      group.clearGroup("a");
      assertThat(group.getGroup("a")).isEmpty();
      assertThat(group.getGroup("b")).containsExactly("b");
   }