@SuppressFBWarnings(value = "EI_EXPOSE_REP", justification = "requiredColumns is ImmutableSet")
  public Collection<? extends ColumnReferenceExp> get() {
    return requiredColumns;
  }
@Test
  public void shouldRemove() {
    // Given:
    builder.addAll(ImmutableSet.of(COL0_REF, COL1_REF, COL2_REF));

    // When:
    builder.remove(COL1_REF);

    // Then:
    assertThat(builder.build().get(), is(ImmutableSet.of(COL0_REF, COL2_REF)));
  }