@Override
  public String toString() {
    return "SingleColumn{"
        + ", alias=" + alias
        + ", expression=" + expression
        + '}';
  }
@Test
  public void shouldCreateSingleColumn() {
    // When:
    SingleColumn col = new SingleColumn(A_LOCATION, AN_EXPRESSION, Optional.of(WINDOWSTART_NAME));

    // Then:
    assertThat(col.toString(), containsString("SingleColumn{, alias=Optional[`WINDOWSTART`], expression='foo'}"));
  }