public List<Column> headers() {
    return byNamespace()
        .get(HEADERS);
  }
@Test
  public void shouldExposeExtractedHeaderColumns() {
    assertThat(SCHEMA_WITH_EXTRACTED_HEADERS.headers(), contains(
        headersColumn(H0, BYTES, Optional.of("key0")),
        headersColumn(H1, BYTES, Optional.of("key1"))
    ));
  }