// build a precomputed version.
    CharMatcher m1 = is('x');
    assertSame(m1, m1.precomputed());
    assertEquals(m1.toString(), m1.precomputed().toString());

    CharMatcher m2 = anyOf("Az");
    assertSame(m2, m2.precomputed());
    assertEquals(m2.toString(), m2.precomputed().toString());

    CharMatcher m3 = inRange('A', 'Z');
    assertSame(m3, m3.precomputed());

    // build a precomputed version.
    CharMatcher m1 = is('x');
    assertSame(m1, m1.precomputed());
    assertEquals(m1.toString(), m1.precomputed().toString());

    CharMatcher m2 = anyOf("Az");
    assertSame(m2, m2.precomputed());
    assertEquals(m2.toString(), m2.precomputed().toString());

    CharMatcher m3 = inRange('A', 'Z');
    assertSame(m3, m3.precomputed());

 * </ul>
 *
 * <h4>Example usage</h4>
 *
 * <pre>{@code
 * class Foo {
 *   private static final ImmutableSet<String> RESERVED_CODES =
 *       ImmutableSet.of("AZ", "CQ", "ZX");
 *
 *   private final ImmutableSet<String> codes;
 *
 *   public Foo(Iterable<String> codes) {
 *     this.codes = ImmutableSet.copyOf(codes);

 * </ul>
 *
 * <h4>Example usage</h4>
 *
 * <pre>{@code
 * class Foo {
 *   private static final ImmutableSet<String> RESERVED_CODES =
 *       ImmutableSet.of("AZ", "CQ", "ZX");
 *
 *   private final ImmutableSet<String> codes;
 *
 *   public Foo(Iterable<String> codes) {
 *     this.codes = ImmutableSet.copyOf(codes);

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