public static Builder builder() {
    return new Builder(ImmutableList.of());
  }
@Test
  public void shouldThrowOnRepeatedHeaderKey() {
    // Given:
    final Builder builder = LogicalSchema.builder()
        .headerColumn(H0, Optional.of("key"));

    // When:
    final Exception e = assertThrows(
        KsqlException.class,
        () -> builder.headerColumn(F0, Optional.of("key"))
    );

    // Then:
    assertThat(e.getMessage(),
        containsString("Schema already contains a HEADER('key') column."));
  }