public Builder asBuilder() {
    return new Builder(columns);
  }
@Test
  public void shouldDetectDuplicateKeysViaAsBuilder() {
    // Given:
    final Builder builder = SOME_SCHEMA.asBuilder();

    // When:
    assertThrows(
        KsqlException.class,
        () -> builder.keyColumn(K0, STRING)
    );
  }