assertThat(is('!').retainFrom("!@#$%^&*()-=")).isEqualTo("!");
    assertThat(is('x').retainFrom("bazaar")).isEqualTo("");
    assertThat(is('a').retainFrom("")).isEqualTo("");
  }

  public void testPrecomputedOptimizations() {
    // These are testing behavior that's never promised by the API.
    // Some matchers are so efficient that it is a waste of effort to
    // build a precomputed version.
    CharMatcher m1 = is('x');