public void set(int idx, SelType obj) {
    val[idx].assignOps(SelOp.ASSIGN, obj);
  }
@Test
  public void testSet() {
    assertEquals("STRING_ARRAY: [foo, bar]", one.type() + ": " + one);
    one.set(1, SelString.of("baz"));
    assertEquals("STRING_ARRAY: [foo, baz]", one.type() + ": " + one);
  }