assertThrows(
        UnsupportedOperationException.class, () -> unmod.computeIfAbsent(3, k -> k + "three"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.computeIfPresent(4, (k, v) -> v));
    assertThrows(UnsupportedOperationException.class, () -> unmod.compute(4, (k, v) -> v));
    assertThrows(UnsupportedOperationException.class, () -> unmod.merge(4, "four", (k, v) -> v));