assertTrue(String.format("contents or modification time of %s have not changed", this), now.modTime != snapshot.modTime || !now.hash.equals(snapshot.hash));
    }

    public void assertContentsHaveChangedSince(Snapshot snapshot) {
        Snapshot now = snapshot();
        assertNotEquals(String.format("contents of %s have not changed", this), snapshot.hash, now.hash);
    }