@Nonnull
    public Vertex localParallelism(int localParallelism) {
        throwIfLocked();
        this.localParallelism = checkLocalParallelism(localParallelism);
        return this;
    }
@Test
    public void when_badLocalParallelism_then_error() {
        v = new Vertex("v", NoopP::new);
        v.localParallelism(4);
        v.localParallelism(-1); // this is good

        Assert.assertThrows(IllegalArgumentException.class, () -> v.localParallelism(-5)); // this is not good
    }