public static Builder builder() {
    return new Builder(new HashSet<>());
  }
@Test
  public void shouldImplementEqualsAndHashCode() {
    new EqualsTester()
        .addEqualityGroup(
            RequiredColumns.builder().add(EXP0).build(),
            RequiredColumns.builder().addAll(ImmutableSet.of(COL1_REF, COL2_REF, COL3_REF)).build()
        )
        .addEqualityGroup(
            RequiredColumns.builder().add(COL0_REF).build()
        )
        .testEquals();
  }