@Override
  public String[] getUsage() {
    if (expression != null) {
      return expression.getUsage();
    }
    return null;
  }
@Test
  public void getUsage() {
    String[] usage = new String[] { "Usage 1", "Usage 2", "Usage 3" };
    when(expr.getUsage()).thenReturn(usage);
    assertArrayEquals(usage, test.getUsage());
    verify(expr).getUsage();
    verifyNoMoreInteractions(expr);
  }