SelType pop() {
    SelType ret = stack[top];
    stack[top--] = null;
    return ret;
  }
@Test
  public void testPop() {
    assertTrue(state.isStackEmpty());
    state.push(SelString.of("foo"));
    state.push(SelString.of("bar"));
    assertFalse(state.isStackEmpty());
    SelType res = state.pop();
    assertEquals("STRING: bar", res.type() + ": " + res);
    res = state.pop();
    assertEquals("STRING: foo", res.type() + ": " + res);
    assertTrue(state.isStackEmpty());
  }