digraph clusters {
    subgraph cluster_0 {
        a -> b -> c -> d -> e
    }

    subgraph cluster_1 {
        a -> f -> c
    }
    
    subgraph cluster_2 {
        rank = same
        p1
        p2
        p3->f
    }
    
    subgraph cluster_3 {
        rank = same
        S1->a
        S2->f
        S3->p1
    }
}

